Methods for forming thin-film heterojunction solar cells from I-III-VI.sub.2

ABSTRACT

An improved thin-film, large area solar cell, and methods for forming the same, having a relatively high light-to-electrical energy conversion efficiency and characterized in that the cell comprises a p-n type heterojunction formed of: (i) a first semiconductor layer comprising a photovoltaic active material selected from the class of I-III-VI 2  chalcopyrite ternary materials which is vacuum deposited in a thin &#34;composition-graded&#34; layer ranging from on the order ot about 2.5 microns to about 5.0 microns (≅2.5 μm to ≅5.0 μm) and wherein the lower region of the photovoltaic active material preferably comprises a low resistivity region of p-type semiconductor material having a superimposed region of relatively high resistivity, transient n-type semiconductor material defining a transient p-n homojunction; and (ii), a second semiconductor layer comprising a low resistivity n-type semiconductor material; wherein interdiffusion (a) between the elemental constituents of the two discrete juxtaposed regions of the first semiconductor layer defining a transient p-n homojunction layer, and (b) between the transient n-type material in the first semiconductor layer and the second n-type semiconductor layer, causes the 
     The Government has rights in this invention pursuant to Contract No. EG-77-C-01-4042, Subcontract No. XJ-9-8021-1 awarded by the U.S. Department of Energy.

The Government has rights in this invention pursuant to Contract No.EG-77-C-01-4042, Subcontract No. XJ-9-8021-1 awarded by the U.S.Department of Energy.

RELATED .[.APPLICATION.]. .Iadd.APPLICATIONS

.Iadd.This is a reissue application of U.S. Pat. No. 4,335,266 which wasissued on June 15, 1982 on application Ser. No. 221,761 filed on Dec.31, 1980..Iaddend.

Reid A. Mickelsen and Wen S. Chen, Ser. No. 278,343, filed July 2, 1981,for "Apparatus for Forming Thin-Film Heterojunction Solar CellsEmploying Materials Selected from the Class of I-III-IV₂ ChalcopyriteCompounds".Iadd., now U.S. Pat. No. 4,392,451 .Iaddend..

BACKGROUND OF THE INVENTION

The present invention relates generally to solar cells and/orlight-to-electrical energy transducers; and, more particularly, to thinlayer p-n-type heterojunction solar cells formed from materials selectedfrom the class of I-III-VI₂ chalcopyrite compounds--especially, a p-n-type heterojunction solar cell having a p-type layer of CuInSe₂ and ann-type layer of CdS--and, to methods of manufacture thereof,characterized in that the cells produced have relatively high energyconversion efficiency characteristics--e.g., closely approximating 10%,or greater--are highly stable, and can be formed with low costfabrication methods on large area, low cost substrates; suchcharacteristics resulting from the formation of a p-n-typeheterojunction device wherein the p-n-type semiconductor layer formedinitially comprises a transient p-n-type homojunction formed of amaterial selected from the class of I-III-VI₂ chalcopyrite compounds inwhich the p-type region of the transient p-type homojunctions is formedby vacuum deposition of a copper-enriched ternary composition or thelike, and the transient n-type region is formed of a copper-deficientternary composition or the like, with a layer of low resistivity n-typesemiconductor material being vacuum deposited on the transient n-typeregion of the first semiconductor layer; whereupon interdiffusion of theelemental constituents in the multi-layer structure causes the transientn-type region of the first semiconductor layer to evolve into a p-typeregion, thereby producing a relatively low resistivity p-n-typeheterojunction device essentially devoid of vacancies, voids, coppernodules and the like which tend to decrease conversion efficiencies;and, thereby enabling the formation of large area, thin film solar cellsutilizing minimal amounts of critical semiconductor materials to form alow cost, stable, polycrystalline thin-film photovoltaic cell on lowcost substrates by the use of low cost fabricating techniques.

Historically, some of the more perplexing problems faced by designers,manufacturers and users of conventional light-to-electrical energytransducers such, for example, as solar cells, have involved the need toimprove: (i) the light energy collection efficiency of the cell; (ii)the conversion efficiency of light into electrical energy; and (iii),the cost involved per unit of power generated to produce such cells.Prior to the recent and continuing "energy crisis", research anddevelopment efforts have been primarily directed to the first two of thethree above-identified factors. As a result, numerous types of solarcells have been designed which have enabled the production of solarcells suitable for use in laboratory experimentation, outer spaceapplications, and the like, wherein the solar cells were relativelysmall area devices--e.g., on the order of 2"×2" --generally formed ofsingle crystals which had to be grown and which were relativelyexpensive. Such crystals are characterized by their lack of grainboundaries; and, are generally limited in size, rarely being larger thanabout 5" in diameter and, usually, being considerably smaller. However,such devices have been known to achieve relatively high energyconversion efficiencies--sometimes ranging in the order of about 14% toabout 16%. While such devices have been highly effective for theirintended purposes, their field of practical use is greatly limited; and,they have simply not been satisfactory for generation of power on apractical economic commercial basis.

With the advent of the recent and continuing "energy crisis", efforts ofresearchers have been redirected; and, a considerable amount of work hasbeen done in attempting to devise various types of energy producingsystems which are: (i) environmentally safe; (ii) not constrained bylimited natural resources; (iii) devoid of the hazards inherent withnuclear energy generating systems; and (iv), capable of producingsufficient energy to meet mankind's ever-increasing energy requirementson a cost-effective basis which is at least competitive with today'scosts for gas, oil, and similar fossil-type fuels or the like. Thepresent invention is believed to constitute the first real step towardsattainment of this long sought-after objective.

PRIOR ART

A typical, but non-exhaustive, list of the types of conventionalphotovoltaic cells which were generated in and prior to the 1960's, andcontinuing into the 1970's, is illustrated by the disclosures containedin U.S. Pat. Nos. 3,186,874 to Gorski, 3,447,234 to Reynolds et al,3,565,686 to Babcock et al, 4,086,101 to Jordan et al, and 4,101,341 toSelders. Briefly, the aforesaid Gorski patent relates to apolycrystalline thin-film CdS/Cu solar cell fabricated by vacuumdeposition of from 20 μm to 100 μm of CdS onto a coated glass substrate,with the coated substrate then being electroplated with copper to form abarrier layer. The CdS layer is preferably doped by adding impurities tothe evaporant powder.

In the Reynolds et al patent, the patentees evaporate CdSe powder on aglass substrate employing indium oxide/gold electrodes. Afterpost-deposition heat treatment in a forming gas, a 20 Å copper overlayis deposited thereon and the device is again heat treated, with thecopper acceptor altering the selenide resistivity, but not the carriertype.

In the Babcock et al patent, the patentees co-evaporate a mixture of CdSand CdSe powders with silver, copper or gold to form a thin-filmphotoconductor in which the metal impurities act as acceptor dopants.

In the Jordan et al patent, a thin-film CdS/Cu_(x) S solar cell isformed on a glass sheet and coated with a transparent tin oxide. The CdSfilm is deposited by spraying a water solution containing a cadmiumsalt, a sulphur compound, and an aluminum containing compound, onto thesubstrate; while the Cu_(x) S layer is formed by a chemical ion exchangeprocess--for example, by dipping or electroplating.

The Selders patent refers to a polycrystalline thin-film heterojunctionsolar cell employing semiconducting selenides of cadmium and tin--i.e.,n-type CdSe and p-type SnSe. Metallization of the device utilizessilver, indium, cadmium, zinc or gold. The device is formed by: (i)evaporation of both the CdSe and SnSe compounds; (ii) evaporation ofCdSe followed by immersion in a tin solution to form SnSe by ionexchange; or (iii), spraying and thermally decomposing solutionscontaining the constituant elements.

Those interested in a comprehensive but non-exhaustive summary of theextensive work that has been carried out in the field of thin-filmheterojunction solar cells, particularly in the 1970's, are referred tothe following articles:

1. Wagner, et al, CuInSe₂ /CdS Heterojunction Photovoltaic Detectors,APPL. PHYS. LETT., Vol 25, No. 8, pp. 434-435 (October 1974).

2. Shay, et al, Preparation and Properties of InP/CdS and CuInSe₂ /CdSSolar Cells, PROC. 11th PHOTOVOLTAIC SPECIALISTS CONF., Phoenix, Ariz.,p. 503 (1975).

3. Wagner, et al, p-InP/n-CdS Solar Cells and Photovoltaic Detectors,APPL. PHYS. LETT., Vol. 26, No. 5, p. 299 (1975).

4. Shay, et al, Efficient CuInSe₂ /CdS Solar Cells, APPL. PHYS. LETT.,Vol. 27, No. 2, pp. 89-90 (July 1975).

5. Tell, et al, Motion of p-n Junctions in CuInSe₂, APPL. PHYS. LETT.,Vol. 28, No. 8, pp. 454-455 (April 1976).

6. Tell, et al, Photovoltaic Properties and Junction Formation inCuInSe₂, J. APPL. PHYS., Vol. 48, No. 6, pp. 2477-2480 (June 1977).

7. Tell, et al, Photovoltaic Properties of p-n Junctions in CuInSe₂, J.APPL. PHYS., Vol. 50, No. 7, pp. 5045-5046 (July 1979).

8. Kazmerski, Ternary Compound Thin Film Solar Cells, FINAL REPORTNSF/RANN/SE/AER 75-19576/PR/75/4, (October 1976).

9. Kazmerski, et al, Thin-Film CuInse₂ /CdS Heterojunction Solar Cells,APPL. PHYS. LETT, Vol. 29, No. 4, pp. 268-270 (August 1976).

10. Kazmerski, et al, Growth and Characterization of Thin-Film CompoundSemiconductor Photovoltaic Heterojunctions, J. VAC. SCI. TECHNOL. Vol.14, No. 1, pp. 65-68 (January/February 1977).

11. Kazmerski, et al, CuInS₂ Thin-Film Homojunction Solar Cells, J.APPL. PHYS., Vol. 48, No. 7, pp. 3178-3180 (July 1977).

12. Kazmerski, Auger Electron Spectroscopy Studies of I-III-VI₂Chalcopyrite Compounds, J. VAC. SCI. TECHNOL., Vol. 15, No. 2, pp.249-253 (March/April 1978).

13. Kazmerski, et al, The Performance of Copper-Ternary Based Thin-FilmSolar Cells, CONF. RECORD, 13th IEEE PHOTOVOLTAIC SPECIALISTS CONF., pp.184-189 (June 5-8, 1978).

14. Kazmerski, et al, Fabrication and Characterization of ITO/CuInse₂Photovoltaic Heterojunctions, CONF. RECORD 13th IEEE PHOTOVOLTAICSPECIALISTS CONF., pp. 541-544 (June 5-8, 1978).

15. Clark, Molecular Beam Epitaxy Research on Copper Indium Diselenide,pp. 385-392, PROC. SOLAR ENERGY RESEARCH INSTITUTE REVIEW MEETING (Oct.10, 1978).

16. White, et al, Growth of CuInSe₂ on CdS Using Molecular Beam Epitaxy,J. APP. PHYS., Vol. 50, No. 1, pp. 544-545 (January 1979).

17. White, et al, Growth of CuInSe₂ Films Using Molecular Beam Epitaxy,J. VAC. SCI. TECHNOL. Vol. 16, No. 2, pp. 287-289 (March/April 1979).

18. Kokubun, et al, Photovoltaic Effect in CuInSe₂ /CdS Heterojunctions,Japan, J. APPL. PHYS., Vol. 16, No. 5, pp. 879-880 (1977).

19. Tomar, et al, ZnCdS/CuInSe₂ and CdS/ZnInSe Thin Film Solar Cells,paper presented at International Electronic Device Meeting, Washington,D.C., (1978).

20. Fleming, Cadmium Sufide/Copper Ternary Heterojunction Cell Research,Paper presented at Solar Energy Research Institute Review MeetingCovering Period from October 1977, through December 1978, pp. 393-420.

21. Piekoszewski, et al Rf-Sputtered CuInSe₂ Thin Films, paper presentedat 14th IEEE PHOTOVOLTAIC SPECIALSTS CONFERENCE, CH 1508-1/80/0000-0980(1980 IEEE).

It should be noted that Reference Nos. (1) through (7) above representwork performed at Bell Telephone Laboratories; while Reference Nos. (8)through (17) represent work carried out under the direction of L. L.Kazmerski at the University of Maine and, later, at the Solar EnergyResearch Institute.

The first reported experimental example of a CdS/CuInSe₂ heterojunctionsolar cell involves the work done at Bell Telephone Laboratories asreported in Reference Nos. (1) through (4), supra. This cell employed asingle crystal of CuInSe₂ and a vacuum deposited CdS film; and,exhibited a uniform photovoltaic quantum efficiency of 70% betweenwavelengths of 0.55 μm to 1.2 μm. For an incident solar intensity of 92mW/cm², the device produced a photocurrent of 38 mA/cm², an open circuitvoltage of 0.49 v, and a conversion efficiency of 12%. Ongoing work byBell Laboratories is described in Reference Nos. (5) through (7), supra.All of the foregoing prior art disclosures pertain to single crystalcells.

Following the development of the single crystal CdS/CuInSe₂heterojunction solar cell by Bell Telephone Laboratories, extensive workwas done by several researchers attempting to produce polycrystallinethin-film cells utilizing vacuum evaporation techniques for both CdS andCuInSe₂. Some of the very early work done in this area is reported inReference Nos. (8) through (10) and (13), supra--work performed at theUniversity of Maine under the direction of L. L. Kazmerski. The CuInSe₂films used in these cells were formed by co-deposition of the CuInSe₂and Se in order to form controlled resistivity p-type layers. Such cellshave demonstrated photocurrents of 28 mA/cm², open circuit voltages of0.49 v, and efficiencies on the order of 6.6% when tested with a lightintensity of 100 mW/cm². Such cells have been approximately 1 cm² inarea and have not been coated with any antireflection layers.

Subsequent reports from the University of Maine (e.g., Reference No. 11,supra) have dealt with CuInS₂ thin-film cells grown by a two-sourcemethod to form a homojunction. The base contact described is zinc/gold,while the top contact is indium. The cell demonstrated a relatively lowconversion efficiency on the order of 3%. In Reference No. 14, supra,there is described an ITO/CuInSe₂ photovoltaic heterojunction whichexhibited a conversion efficiency on the order of 8.5% in a singlecrystal device, but only 2.0% in a polycrystalline device.

Reference Nos. (15) through (17), supra, are illustrative of methods forforming CuInSe₂ thin-film cells utilizing Molecular Beam Epitaxy ("MBE")systems. However, as well known to those skilled in the art, MBE systemsare simply not consistent with the demand for low-cost, large area solarcells; but, rather, are confined principally to the development ofrelatively small single crystal cells suitable for laboratory and/orexperimental purposes.

In Reference No. (18), supra, Kokubun reports on the photovoltaic effectin a CuInSe₂ /CdS heterojunction solar cell employing an evaporated goldohmic contact on the photovoltaic material and demonstrating anefficiency of 5.6%. Tomar et al and Fleming (Reference Nos. 19 and 20,supra,) each report on p-type CuInSe₂ ternary heterojunction thin-filmsolar cells in which the semiconductor layers are deposited byevaporation techniques; whereas, Piekoszewski, et al (Reference No. 21,supra) reports on a similar cell wherein the CuInSe₂ thin-films aredeposited by Rf-Sputtering techniques. In this latter case, the reportedefficiency of the cell was on the order of 5%.

An overall general review of the state of the foregoing prior art hasbeen set forth by Wagner, et al, Multi-component Tetrahedral CompoundsFor Solar Cells, J. CRYSTAL GROWTH, Vol. 39, pp. 151-159 (1977), whereinthe authors provide an overall review of the use of chalcopyrite-typesemiconductors and the development of the high efficiency single crystalCuInSe₂ /CdS cell by Bell Laboratories and the thin-film CuInSe₂ /CdScells developed at the University of Maine. While this review reports onmany different types of solar cells and the constituent materials fromwhich they have been made, in general it has been found that there havebeen only four combinations of materials which have been utilized andwhich have provided conversion efficiencies exceeding 10%--viz., (i)silicon; (ii) GaAs/Ga_(x) Al_(1-x) As; (iii) InP/CdS; and (iv), CuInSe₂/CdS--and, in each and every instance, prior to the advent of thepresent invention, those devices described that did exceed a 10% energyconversion figure were in single crystal form--i.e., a form whichinherently precludes the use of low cost fabrication techniques and theapplication of thin-layer films on large area substrates (See, e.g.,Reference No. 10, supra, p. 65). On the other hand, conversionefficiencies of less than 10% and, particularly, in the range of fromabout 2% to about 7%--the range most commonly attained in the priorart--require such tremendous support systems and dedication of groundspace that the systems are neither viable nor practical from acommercial energy producing standpoint, even where they meet therequisite of low cost. Therefore, one of the principle directions inwhich researchers have been extending their efforts has been towards theformation of thin-layer, heterojunction solar cells preferably formed ofchalcopyrite materials which are suitable for application to large areasubstrates--i.e., which are capable of approximating and, preferably,exceeding 10% conversion efficiencies.

But, prior to the advent of the present invention, not one of thethin-layer, heterojunction, polycrystalline-type, large area solar cellsreported on have begun to approach conversion efficiencies on the orderof 10%; including a p-type CuInSe₂ and n-type CdS heterojunction solarcell described by Reid A. Mickelsen and Wen S. Chen in an articleentitled High Photocurrent Polycrystalline Thin-Film CdS/CuInSe₂ SolarCell, APP. PHYS. LETT., Vol, 36, No. 5, pp. 371-373 (1980) wherein themaximum conversion efficiency attained was found to be 5.7% undersimulated 100 mW/cm² solar ilumination.

One problem that has been repeatedly faced by researchers involves theconflicting characteristics of chalcopyrite materials which are, on theone hand, sometimes low resistivity p-type chalcopyrite materials and,on the other hand, either n-type chalcopyrite materials or highresistivity p-type chalcopyrite materials. That is, in the case of lowresistivity p-type chalcopyrite materials which are exposed to CdS,researchers have been continually plagued by the creation of a highresistivity region in the CdS layer, generating voids and vacancies inthe chalcopyrite semiconductor materials which commonly take the form ofcopper nodules. Such nodules, are highly undesirable, either serving toincrease the resistivity of the n-type CdS layer or forming largedefects and resulting in significantly lower conversion efficiency. Atthe same time, however, it has been known by persons skilled in the artthat the formation of undesirable copper nodules is not prevalent whenusing either high resistivity p-type chalcopyrite materials or n-typechalcopyrite materials. Unfortunately, however, usage of highresistivity p-type chalcopyrite materials has tended to lead to rapiddegradation of the cell with such cells being characterized by theirlack of stability and somewhat low conversion efficiencies. Of these twoproblems, researchers have found that the problems produced by coppernodules are of such a magnitude that the prior art has generally tendedto develop thin-film, large area cells fabricated from relatively highresistivity p-type chalcopyrite semiconductor materials with theconsequent result that conversion efficiencies have generally plateauedin the region of from about 5% to about 7%.

SUMMARY OF THE INVENTION

Accordingly, it is a general aim of the present invention to provideimproved light transducers such, for example, as solarcells--especially, large area, thin-film heterojunction cells formedfrom materials selected from the class of I-III-VI₂ chalcopyritecompounds--and to provide improved methods for forming such cells, whichovercome the foregoing disadvantages inherent with prior art thin-filmtransducers and manufacturing processes and which permit the formationof large area thin-film cells characterized by their high energyconversion efficiencies--efficiencies approaching on the order of 10%,or greater--and, wherein the cells do not exhibit excessive voids,vacancies and/or the formation of copper nodules in the semiconductorlayers.

In one of the its principal aspects, it is an object of the invention toprovide improved methods for forming large area, low cost, stable,polycrystalline, thin-film photovoltaic cells on low cost substratematerials utilizing low cost fabricating methods.

An ancillary object of the invention is the provision of improvedmethods for forming low cost thin-film solar cells which, for the firsttime, are economically competitive with more conventional gas, oil andsimilar fossile fuel type energy generating systems, as well as withmore exotic nuclear energy generating systems, and which readily permitthe formation of such improved cells on a large scale production basis,yet wherein the solar cells produced are environmentally safe andessentially hazard-free.

A further objective of the invention is to provide improved low costthin-film cells, and methods for manufacture thereof, wherein the cellsare characterized by their stability and wherein energy conversionefficiency does not degrade even after months of storage, and evenwithout encapsulation.

In another of its important aspects, it is an object of the invention toprovide an improved p-n-type heterojunction device formed of materialsselected from the class of I-III-VI₂ chalcopyrite materials wherein thesemiconductor materials in the p-type region of the p-n-typeheterojunction initially define a high resistivity transient n-typelayer deposited upon the initial p-type layer and, wherein such highresistivity transient n-type layer ultimately evolves (byinterdiffusion) into a high resistivity p-type layer; and, wherein theinitially deposited low resistivity p-type layer is characterized by:its high adherence; low contact resistance; a back surface field effect;large, relatively uniform grain size; and, moreover, the low resistivityp-type layer acts as an important source for electrically active speciesto diffuse into subsequent film layers.

Another general objective of the invention is to provide an improvedp-n-type heterojunction device characterized by a p-side beingpredominantly single phase chalcopyrite structure.

A further objective of the invention is the provision of improvedmethods for forming large area thin-film heterojunction solar cellswhich readily permit of reproducibility of cells having desired energyconversion efficiencies.

Briefly stated, the foregoing objectives are attained by forming ap-n-type heterojunction wherein the p-type semiconductor layercomprises: (i) a low resistivity p-type material; and (ii), a relativelyhigh resistivity transient n-type material having the same elementalcomposition--preferably, CuInSe₂ --but, employing differing ratios ofthe elemental constituents copper and indium (or other type I-IIIelements), thereby defining a transient p-n-type homojunction; and,thereafter, vacuum depositing thereon a low resistivity n-typesemiconductor material--preferably an indium-doped CdS layer--whereuponinterdiffusion between the transient high resistivity n-type layer andthe adjacent low-resistivity p-type and n-type layers causes thetransient n-type layer and the transient homojunction to evolve into acomposition graded p-type layer, thereby producing a p-n-typeheterojunction device essentially devoid of excessive voids, vacanciesand copper nodules and which is characterized by relatively high energyconversion efficiency.

DESCRIPTION OF THE DRAWINGS

These and other objects and advantages of the present invention willbecome more readily apparent upon reading the following detaileddescription and upon reference to the attached drawings, in which:

FIG. 1 is a highly diagrammatic plan view, somewhat enlarged in scale,here depicting a fragmentary portion of a large area thin-film cellwhich is here representative of the general external visual appearanceof both conventional thin-film cells and cells made in accordance withthe present invention, here depicting the current collecting electrodesin highly exaggerated spread form, it being understood that in actualitythe electrodes would appear as relatively closely spaced, generallyparallel, fine lines on the order of 25 μm in width;

FIG. 2 is a fragmentary, highly diagrammatic sectional view of athin-film cell embodying features of the present invention, here takenfor purposes of illustration substantially along the line 2--2 in FIG.1, it being understood that the dimensional relationships illustratedare in exaggerated form for purposes of clarity and that in actualitysuch a cell will normally have an overall thickness on the order of onlyabout 5.0 μm;

FIG. 3 is a diagrammatic block-and-line representation of a process asheretofore commonly employed for forming, for example, conventionalprior art thin-film CdS/CuInSe₂ heterojunction cells;

FIG. 4 is a view similar to FIG. 3, but here illustrating a step-by-stepprocess for forming large area thin-film heterojunction cells inaccordance with the present invention;

FIG. 5 is a highly diagrammatic view here depicting in verticalsectional format the discrete laminations forming cells manufactured inaccordance with the present invention with representative and preferredprocess temperature and product thickness parameters set forth for eachdifferent lamination;

FIG. 6 is a highly diagrammatic vertical sectional view of an exemplarysystem configuration for preparing thin-film CuInSe₂ films on solar cellsubstrates, it being understood that the system here depicted forillustrative purposes only is commonly the type of system used inlaboratory work for forming relatively small cells which may, forexample, be on the order of 1-10 cm²,

FIG. 7 is a highly magnified (2000×) microphotograph of a typical lowresistivity CuInSe₂ film formed in accordance with conventional priorart methods and taken at an oblique angle to the surface of the cell,here depicting particularly the resulting copper nodules that are formedwith such materials; even in the presence of only minimal amounts ofCdS;

FIG. 8 is a highly magnified (2000×) microphotograph taken at an obliqueangle to the surface of the solar cell, here detpicting the cell shownin FIG. 7 after application of a low resistivity n-type CdSsemiconductor layer thereon;

FIG. 9 is a highly magnified (2000×) microphotograph taken at an obliqueangle to the surface of the solar cell and similar to the presentationin FIG. 7, but here illustrating a first "composition-graded" CuInSe₂layer vacuum deposited on the substrate in accordance with the presentinvention and illustrating particularly the absence of copper nodules;

FIG. 10 is a microphotographic view of a portion of the surface depictedin FIG. 9, here shown at 5000× magnification;

FIG. 11 is a graphic presentation illustrating efficiency of a thin-filmheterojunction cell made in accordance with the present invention as afunction of time following heat treatment, efficiency being illustratedon the ordinate and time (in days) on the abscissa;

FIG. 12 is a graphic presentation of the photovoltaic characteristics ofa high efficiency cell made in accordance with the present invention andsubjected to simulated AM1 (101.5 mW/cm²) illumination with currentdensity (mA/cm²) illustrated on the ordinate and voltage (v) illustratedon the abscissa, here depicting the photovoltaic characteristics bothwithout an antireflection coating applied to the cell and afterapplication of an antireflection coating to the cell;

FIG. 13 is a graphic presentation illustrating the quantum efficiency asa function of wavelength for the high efficiency cell of the presentinvention, here depicting quantum yield (electrons/photon) on theordinate and wave-length (micrometers) on the abscissa;

FIG. 14 is a graphic representation of the experimental and calculatedvalues of fill factor (the ordinate) as a function of J_(L) /J_(o) (theabscissa) for a high efficiency cell embodying the features of thepresent invention;

FIG. 15 is a highly diagrammatic elevational block-and-line diagramillustrating an exemplary continuous in-line production system forforming CdS/CuInSe₂ heterojunction, thin-film, large area solar cells inaccordance with the present invention and,

FIG. 16 is a fragmentary, highly diagrammatic sectional view of amodified thin-film cell similar to the form of the invention depicted inFIG. 2, but here comprising an n-p-type heterojunction embodyingfeatures of the present invention as contrasted with the p-n-typeheterojunction shown by way of example in FIG. 2.

While the invention is susceptible of various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and will herein be described in detail. Itshould be understood, however, that it is not intended to limit theinvention to the particular forms disclosed but, on the contrary, theintention is to cover all modifications, equivalents and alternativesfalling within the spirit and scope of the invention as expressed in theappended claims.

DETAILED DESCRIPTION

Turning now to FIGS. 1 and 2 conjointly, a fragmentary portion of anexemplary thin-film p-n-type heterojunction solar cell, generallyindicated at 30, has been diagrammatically illustrated. As the ensuingdescription proceeds, it will become apparent that the illustrativesolar cell 30 may readily be formed with low cost, large area,fabrication techniques on low cost, large area substrate materials.Nevertheless, such a cell may also be formed on an experimentallaboratory basis in relatively small area sizes--say, for example, 2cm×2 cm. Dependent upon the particular end results desired, thethicknesses of the various layers of the cell 30 may vary somewhat; but,in general, thin-layer heterojunction cells of the type to be describedherein will commonly have an overall thickness (excluding the thicknessof the substrate material which may be on the order of approximately0.025") ranging from about 5.0 μm to about 10.0 μm. Such thin-film cellsare to be contrasted with the more conventional single crystal,silicon-type solar cells and concentrator solar cells wherein thethickness of the active portions of the cell may range from 200 μ m to400 μm.

The exemplary cell depicted in FIGS. 1 and 2 is representative, at leastin visual appearance, of both thin-film cells embodying the features ofthe present invention and cells formed of similar materials, but made inaccordance with the processes known in the prior art. See, e.g.,Reference Nos. (8) through (21), supra. In either case, the activelayers of the cell are deposited on a suitable substrate 31, which ishere preferably formed of aluminum oxide (Al₂ O₃) having a thickness onthe order of 0.025". While the highest efficiencies achieved with cellsmade in accordance with the present invention have been attainedutilizing cells having Al₂ O₃ substrates 31 in the form of apolycrystalline alumina, it is believed that other materials can also beused including, for example, glazed alumina, enameled steel, metalfoils, and similar inert inorganic materials, provided only that thematerial selected is capable of withstanding the process temperaturesinvolved which range up to about 500° C.

A suitable base contact 32 is applied directly to one surface of thesubstrate 31. Again, in carrying out the present invention we have foundthat excellent results are attained when using molybdenum (Mo); but,again, it is believed that other materials could be used withoutdeparting from the spirit and scope of the invention. For example, amolybdenum/gold material could be used and, perhaps, other materialssuch as the more conventional nickel and graphite materials which havebeen commonly employed in conventional solar cells.

The essence of any light-to-electrical energy transducer, be it aconventional prior art-type solar cell or a solar cell made inaccordance with the present invention, lies in the photoactivesemiconductor materials defining the junction, generally indicated at 34in FIG. 2, which may comprise a p-n-type junction or an n-p-typejunction of either the homojunction variety (wherein the junction isformed by impurity diffusion or implantation into one surface of thephotoactive semiconductor material), or of the heterojunction variety(wherein the junction is defined at the interface between two differentsemiconductor materials, one of which is an n-type material and theother of which is a p-type material). The present invention relates tothis latter type of junction device--viz., a heterojunction device--hereconsisting of first and second respectively different semiconductorlayers 35, 36. Moreover, while it is believed that the present inventionmay well find applicability with n-p-type heterojunction devices,particularly in the case of non-solar cell applications having band gapenergies of greater than 1.5 ev, it will herein be described inconnection with an exemplary p-n-type heterojunction 34 suitable for useas a solar cell having a relatively narrow band gap energy ranging fromabout 1 to 1.5 ev, and suitable for use as a solar cell having a firstsemiconductor layer 35 formed of p-type material and a secondsemiconductor layer 36 formed of n-type material.

Historically, a wide range of photovoltaic materials have been used withvarying degrees of success in the formation of solar cellsemiconductors. Recently, however, the potential utilization of ternarychalcopyrite compounds (see, e.g., Reference Nos. 1-21, supra) asalternatives to the more conventional photovoltaic materials has beendemonstrated; and, indeed, the desirability of utilizing materialsselected from the class of I-III-VI₂ chalcopyrite compounds has stirredparticular interest.

Properties of potential solar cell materials selected from the class ofI-III-VI₂ chalcopyrite compounds have been set forth in Table I. Suchmaterials all have band gaps near the optimum value for terrestrialsolar energy conversion. They are direct band gap semiconductors whichminimize the requirement for long minority carrier diffusion lengths.Except for CuGaSe₂, which has only exhibited p-type behavior, the otherthree ternary compounds have been grown as both n-type and p-typecrystals. Thus, construction of homojunction devices is possible. Thesechalcopyrite compounds can also be paired with CdS to potentially makeefficient p-n-type heterojunction solar cells because they havecompatible lattice structures with acceptable lattice mismatches, andfavorable differences of electron affinities. When used in applicationsother than solar cells, other I-III-VI₂ compounds may be employed such,for example, as: AgInS₂ ; AgGaSe₂ ; AgGaTe₂ ; AgInSe₂ ; CuGaTe₂ ;AgInTe₂ ; and CuGaS₂.

                                      TABLE 1                                     __________________________________________________________________________    PROPERTIES OF POTENTIAL SOLAR CELL I-III-VI.sub.2 Materials                                                Mobility                                                         Lattice                                                                              Mismatch                                                                            (cm.sup.2 V.sup.-1 S.sup.-1)                                                          Electron                                 Eg(ev)          Constant (Å)                                                                     With CdS                                                                            (300° K.)                                                                      Affinity                                 Material                                                                           (300° K.)                                                                   Transition                                                                          a  c   (%)   n   p   (ev)                                     __________________________________________________________________________    CuGaSe.sub.2                                                                       1.68 direct                                                                              5.618                                                                            11.01                                                                             3.8   --  20  --                                       CuInS.sub.2                                                                        1.55 direct                                                                              5.523                                                                            11.12                                                                             5.56  200 15  --                                       CuInSe.sub.2                                                                       1.04 direct                                                                              5.782                                                                            11.62                                                                             1.16  320 10  4.58                                     CuInTe.sub.2                                                                       0.96 direct                                                                              6.179                                                                            12.36                                                                             5.62  200 20  --                                       CdS  2.42 direct                                                                              4.136                                                                            6.716                                                                             --    250 --  4.5                                      __________________________________________________________________________

These materials, in general, have band gap energies outside the range of1-1.5 ev most desirable for solar cells and, in addition some haveexhibited only n-type behavior (e.g., AgInS₂ and AgInSe₂) while AgGaSe₂is only suitable as a high resistivity photovoltaic material.

Reported performances of solar cells based on ternary compounds selectedfrom the class of I-III-VI₂ chalcopyrite compounds are set forth inTable II. The high efficiency of the single crystal device described inReference Nos. (1) through (4), supra, as well as the polycrystallinenature of the exemplary thin-film devices here under consideration, hasled to the description herein of a preferred form of the inventionemploying a CuInSe₂ /CdS photovoltaic p-n-type semiconductor junction34; and, such devices have been found to provide excellent low costpower generation systems capable of operating at practical and viablepower conversion efficiencies which are "cost-competitive" with the moreconventional fossil fuel power generating systems, as well as with themore exotic nuclear energy generating systems, all as herein describedin connection with the present invention.

                  TABLE II                                                        ______________________________________                                        REPORTED PHOTOVOLTAIC EFFICIENCY                                                           Efficiency (%)                                                   Material       Single Crystal                                                                            Thin-Film                                          ______________________________________                                        CdS/CuGaSe.sub.2                                                                              5          --                                                 CdS/CuInS.sub.2                                                                              --          3.25                                               CdS/CuInSe.sub.2                                                                             12          6.6, 5.7                                           CdS/CuInTe.sub.2                                                                             No significant photovoltaic effect                             n, p CuInS.sub.2                                                                             --          3.6                                                n, p CuInSe.sub.2                                                                            --          3.0                                                ______________________________________                                    

Finally, in order to complete the exemplary cell 30 depicted in FIGS. 1and 2, there is provided a suitable grid contact pattern which isdeposited on the upper surface of the semiconductor layer 36 defining aportion of the junction 34. In the illustrative cell depicted at 30 inFIGS. 1 and 2, the contact grid comprises a plurality of generallyparallel, fine-line electrodes 38 which are electrically connected to asuitable current collecting electrode 39 (FIG. 1) which is hereillustrated as being disposed at, or adjacent to, the edge of thephotoactive region of the cell; but which may, if desired, be depositedon top of the photoactive region of the cell. Such positioning does,however, reduce the optical transparency of the system. The grid-typeelectrodes 38, 39 may be formed of various materials; but, it isessential that such materials be characterized by their highconductivity and their ability to form a good ohmic contact with theunderlying semiconductor layer 36. In the practice of the presentinvention, excellent results have been attained utilizing aluminum whichreadily permits of through-mask vacuum deposition with conventionalfabricating techniques and which is characterized by high conductivitycharacteristics and excellent ohmic contact characteristics,particularly when the underlying semiconductor comprises CdS or thelike. However, other materials might be used such, for example, asindium, chromium, molybdenum and similar materials to form an interfaceproviding the desired ohmic contact characteristics with a superimposedelectrode material such, for example, as copper, silver, nickel or thelike.

To improve the light collection efficiencies of the solar cell 30, thelaminated thin-film device herein described is conventionally providedwith an antireflective coating 40 in a manner well known to personsskilled in the art. Again, while the particular material used to formthe antireflective coating 40 is not critical to the present invention,excellent results have been attained when using an antireflectivecoating formed of SiOx--a suboxide of silicon wherein "x" varies between"1" and "2" dependent upon the deposition parameters employed.Particularly excellent results have been achieved when the value of "x"is on the order of about "1.8". However, as indicated, other materialscan be utilized including, for example, SiO₂, aluminum oxide, tantalumoxide, etc., although preferably the antireflective coating will beselected from a material having a refraction index of about 1.55.

Referring next to FIG. 3, there has been diagrammatically illustrated inblock-and-line form a typical step-by-step process for forming athin-film heterojunction device of the types disclosed in Reference Nos.(1) through (21), supra; and, particularly, those conventional prior artsolar cells formed utilizing a ternary chalcopyrite material for onesemiconductor layer and CdS as the second semiconductor layer, with thetwo layers defining a heterojunction. Thus, as here shown, in step (a) asuitable base contact 32 is applied to substrate 31 in any of the wellknown conventional manners such, for example, as by Rf-Sputteringtechniques, vacuum deposition, or the like. Thereafter, the firstsemiconductor layer 35, which is here shown as a ternary chalcopyritecompound and, more particularly, as CuInSe₂, is then applied to the basecontact 32 during step (b), generally by vacuum deposition techniques.

Following application of the first semiconductor layer 35, the secondsemiconductor layer 36, which is here shown for illustrative purposes tobe CdS, is preferably vacuum deposited in step (c) on the firstsemiconductor layer 35, with the two layers 35, 36 defining aheterojunction type device 34. Thereafter, the grid contact arrangement38, 39 is applied to the surface of the upper semiconductor 36 duringstep (d); conventionally, by means of through-mask evaporationtechniques. Finally, an antireflective coating 40 is applied to theupper surface of the cell over the grid contact pattern and the exposedportions of the semiconductor layer 36 during step (e). As those skilledin the art will appreciate, the conventional process parameters employedin terms of vacuum parameters, temperature parameters, and/orintermediate heating steps have been eliminated from the abovediscussion simply for purposes of clarity; but, it will be understoodthat the conventional process would include utilization of such processparameters.

Turning now to FIG. 4, and simply for purposes of facilitating a broad,general, preliminary understanding of the differences between theprocess of the present invention and the conventional process shown inFIG. 3, there has been illustrated in step-by-step, block-and-line forman exemplary process preferably utilized to form thin-filmheterojunction solar cells in accordance with the present invention. Ashere illustrated, the base contact 32 is applied directly to the surfaceof the substrate 31 in step (a) in a manner which can be essentially thesame as that described in step (a) of the prior art process depicted inFIG. 3. In the formation of experimental laboratory type thin-filmheterojunction solar cells embodying the features of the presentinvention, such application has been by Rf-Sputtering techniques whereinthe substrate is neither heated or cooled but, is generally disposed ona water cooled platen. Those skilled in the art will appreciate thatsuch Rf-Sputtering techniques generally generate considerable heat,serving to heat the substrate 31 by several hundred degrees.

The principal process difference between the methods embodying featuresof the present invention and those utilized in the prior art resides inthe process parameters employed to form the thin-film heterojunction 34.More specifically, in the practice of the present invention, the firstsemiconductor layer 35 is applied by a vacuum deposition technique and,during the vacuum deposition process the copper/indium ratio iscarefully controlled in a manner to be hereinafter described in greaterdetail. That is, during the first portion of the vacuum depositionprocess, the copper/indium ratio in the ternary CuInSe₂ compound iscontrolled to provide a slightly copper-enriched composition. Morespecifically, as contrasted with a stoichiometric composition whereinthe CuInSe₂ is neither a p-type nor an n-type material, in the initialportion of the process for applying the semiconductor 35, a slightcopper excess on the order of about 5% to 10% is provided so as to forma first region 35a during step (b₁) which is basically a low resistivityp-type semiconductor region. At that point in the process when thethickness of the semiconductor layer 35 is generally on the order ofbetween 50% and 66.7% of the desired total thickness, the copper/indiumratio is adjusted so that the ternary material being applied during thestep (b₂) in the vacuum deposition process is slightlycopper-deficient--e.g., on the order of up to about 5%copper-deficient--so as to form a moderately high resistivity n-typeregion 35b which is deposited directly upon the copper-enriched region35a. Thus, the two regions 35a and 35b of semiconductor layer 35 definea composition gradient in the ternary chalcopyrite materials from whichsemiconductor 35 is formed; and, indeed, the two regions 35a, 35b definea transient p-n-type homojunction. Such p-n-type homojunction is termed"transient" because the copper deficient region 35b tends to be amoderately high resistivity transient n-type region which, throughinterdiffusion with respect to its adjacent region 35a and layer 36,evolves into a p-type region, thereby defining a "composition-graded"p-type semiconductor layer 35.

In carrying out the present invention, the uppermost semiconductor layer36 in the exemplary device is an n-type layer and, preferably, an n-typeCdS semiconductor layer. Desirably, this layer 36 is a low resistivitylayer; and, to achieve this desired result, the CdS layer 36 is vacuumdeposited on the first semiconductor layer 35 in a carefully controlledprocess wherein CdS is deposited during step (c₁) to a depth generallyon the order of about 0.8 μm and, thereafter, the CdS deposited in theremaining thickness of layer 36 is indium-doped to insure lowresistivity characteristics. Thus, the layer 36 comprises a underlyingCdS region 36a which is essentially undoped and an overlyingindium-doped region 36b.

Steps (d) and (e) for respectively applying the grid contacts 38, 39 andthe antireflective coating 40 are, for purposes of an understanding ofthe broader aspects of the present invention, essentially similar tosteps (d) and (e) described above in connection with FIG. 3.

Turning next to FIG. 5, there has been diagrammatically illustrated anexemplary thin-film heterojunction device formed of materials selectedfrom the class of I-III-VI₂ chalcopyrite compounds and which resultsfrom the process hereinabove described generally in connection with FIG.4. Thus, as here illustrated, the polycrystalline thin-film CuInSe₂ /CdScells made in accordance with the present invention are prepared onmetallized alumina (Al₂ O₃) substrates 31 which are generally on theorder of about 0.025" thick by deposition thereon of a thin metalmolybdenum (Mo) film or the like--for example, a film on the order ofabout 6000 Å in thickness--such film having been applied in anexperimental laboratory procedure by Rf-Sputtering techniques suitablefor forming a low cost metallization for the cells. The Mo layer 32 hasbeen found to be stable, of low resistivity--0.2Ω/□ for films of 6000 Åthickness--highly adherent, and to have formed excellent ohmic selenidecontacts. Sputtering was done in 6 μm of argon with a power of 10.0W/in.² for about 60 minutes.

In keeping with the important aspects of the present invention, theexemplary p-n-type heterojunction 34 is then applied in the mannerdescribed above in connection with FIG. 4. More specifically, the firstsemiconductor layer 35 is deposited in two discrete superimposed regions35a, 35b with the overall thickness of the layer 35 preferably being onthe order of approximately 3.5 μm. Region 35a, which is preferably inthe range of 1.0 μm to 3.0 μm, is the ternary chalcopyrite compounddeposited by simultaneous elemental evaporation at a temperature on theorder of about 350° C.; although, less preferable higher temperaturesranging up to about 500° C. can be employed. In the laboratoryenvironment wherein the illustrative photocell 30 was initiallyfabricated, the region 35a was deposited to a desirable thickness duringa period of 40 minutes; such thickness comprising in the range of from50% to 66.7% of the overall desired thickness for layer 35.

At the completion of that time, and further in accordance with theinvention, the copper/indium ratio was then adjusted during thesimultaneous elemental evaporation process so as to provide asimultaneous elemental vapor stream that was slightly copper-deficient,thereby forming the copper-deficient, moderately high resistivity,transient n-type region 35b. During the course of this simultaneouselemental evaporation process which lasted for 20 minutes in thelaboratory environment herein described, the temperature parameter wasraised from 350° C. to approximately 450° C. during the last quarter ofthe overall evaporation process used to deposit layer 35--i.e., at aboutthe 45 minute mark or, about 5 minutes after adjusting the copper/indiumratio from a copper-enriched ternary compound to a copper-deficientternary compound. While excellent results have been achieved utilizingtemperature parameter on the order of 450° C. during the last quarter ofthe foregoing evaporation step, it has been found that the temperaturemay fall generally in the range of 450°± and 25° C., but it has beenfound that the temperature should be maintained at less than about 500°C.

At this point in the process, the second semiconductor layer 36--here,preferably, a low resistivity n-type CdS layer--is applied to thepreviously applied composition-graded transient p-n homojunction formedby semiconductor layer 35. Preferably, the low resistivity n-type layer36 is on the order of about 3 μm in thickness, consisting of a firstundoped CdS region 36a ranging in thickness from about 0.5 μm to about1.5 μm, and a superimposed indium-doped region 36b ranging in thicknessfrom about 2.0 μm to 4.0 μm. To this end, the process temperature wasdecreased in the laboratory experiment to a temperature ranging between150° C. and about 200° C. The solar cell produced having the highestenergy conversion efficiency--an efficiency of ≅9.53%--was preparedutilizing a temperature of 200° C. during the CdS vaporization process.At the lower temperature of 150° C., sheet resistivity for CdS filmsranging in thickness from 3.0 μm to 5.0 μm was in the range of 60-200KΩ/□. In the exemplary cells, after deposition of approximately 0.8 μmof pure CdS, the CdS films were doped with indium (≅1.5%) byco-evaporation. Such doping formed a very low resistivity region (30-100Ω/□) in contact with the subsequently deposited grid structure 38, 39.

Chamber pressure during all selenide depositions was maintained at3-8×10⁻⁶ torr.

In keeping with the invention, the grid contacts 38, 39 (FIGS. 1, 2 and5) were applied on top of the CdS semiconductor layer 36 utilizingconventional through-metal mask techniques and an evaporation systememploying an electron gun source (not shown) for aluminum deposition.The grid lines or electrodes 38 are preferably on the order of about 2.0μm in thickness and are extremely fine electrode lines ranging in widthfrom about 25 μm to 50 μm. The exemplary laboratory solar cells wereformed utilizing grid lines of approximately 25 μm in width with tenequally spaced parallel lines per centimeter, defining a transparentgrid structure exposing from 93% to 95% of the underlying semiconductorlayer 36. Finally, an SiO_(x) antireflective coating (where "x" is equalto approximately "1.8") was applied by vacuum evaporation attemperatures ranging from 100° C. to 125° C.

In the formation of thin-film heterojunction solar cells in accordancewith the present invention, it has been found that the electricalproperties of the CuInSe₂ are extremely sensitive to the copper/indiumratio. Indeed, it has been found that variation in that ratio of only afew percentage points results in resistance gradients of 10⁴ to 10⁵.Consequently, by simply adjusting the relative evaporation rates of thecopper and indium, it is possible to achieve the desired filmproperties. It has also been found that control of the selenium is notcritical.

Referring now to FIG. 6, there has been illustrated an exemplarylaboratory system for forming heterojunctions 34 embodying the featuresof, and made in accordance with the methods of, the present invention.As here shown, the system employs a conventional enclosed vacuumchamber, diagrammatically depicted at 50. The substrate 31 is positionedbetween a shutter 51 and a suitable heating device 52 such, for example,as a carbon cloth substrate heater. The metallic elements (i.e., typeI-III elements such as copper and indium) for the ternary chalcopyritecompound--e.g., CuInSe₂ --are positioned within a crossed boat sourceconfiguration for copper and indium vaporization. Thus, the indiumsource is deposited within a first boat 54 having a carbon block 55positioned centrally within the boat to form two sources of indiumvapor, one on either side of the block 55. Positioned above the carbonblock 55 and oriented at right angles to the boat 54 is a second boat 56containing the copper source. In the laboratory experimental set-up,both boats: (i) were made of tungsten; (ii) contained an aluminabarrier; and (iii) were obtained from R. D. Mathis Company, Long Beach,Calif. Boat widths of 1/2 " and 3/4" for copper and indium,respectively, as well as 3/4" for both materials, were found acceptable.The vertical spacing between the boats 54, 56 was approximately 1/8".The selenium source was contained within a pair of boats 58, 59installed at opposite ends of, and below, the substrate 31 to insuredeposition uniformity. However, a single selenium source has alsoproduced satisfactory results.

In carrying out the present invention, a dual-channel co-evaporationcontroller employing the principals of Electron Impact EmissionSpectroscopy (EIES) was provided for both monitoring and controlling theindium/copper ratio. In the experimental system, the dual-channelco-evaporation controller was a model manufactured by Inficon, locatedin New York, and identified as Inficon's model "Sentinel 200". Whilesuch EIES systems are well known to persons skilled in the art--see, forexample, Schumacher U.S. Pat. No. 3,612,859--and need not be describedin detail, a brief description may be of some assistance in facilitatingan understanding of the present invention. With this system, the sensor60 of the EIES controller (FIG. 6) was positioned so as to permitevaporated materials in the vapor stream emanating from the crossedboats 54, 56 to enter the miniature sensor structure (not shown) whereinthe evaporant is cross-bombarded by a low energy electron beam. Afraction of the atoms are excited during the collision process. In theexcited state, the outer shell electrons of these atoms are raised toenergy levels higher than the normal ground state. Almost immediately,most of the excited atoms cascade to lower energy states, emittingphotons with specific energies or wavelengths. These wavelengths aregenerally in the 2000 Å to 4500 Å ultraviolet light region and areprecisely characteristic of the atomic species. The number of photonsemitted (light intensity) is proportional to evaporant density in thesensor so that light intensity is then proportional to evaporation rate.By the use of narrow band pass optical filters and/or monochrometers(not shown), two specific materials can be simultaneously monitored andrate controlled. In the illustrative system, the EIES controller wasequipped with an optical band pass filter (4500 Å) on one channel tomonitor indium, and a monochrometer set at 3250 Å on the other channelto monitor copper. The sensor 60 was mounted on the chamber bell-jar 50and positioned above, but off axis from, the crossed boat configuration54, 56.

While the EIES sensor was used to monitor and control evaporation ofindium and copper, a quartz crystal microbalance deposition controller61 was provided for controlling the selenium vaporization rate from theboats 58, 59. Suitable shields (not shown) were provided to preventexposure of the EIES controller to selenium. Of course, the particularsensor/controller employed can vary dependent upon specific systemrequirements. For example, the system may employ a Mass Analyzer or aquadrupole-type analyzer, etc. But, we have found that excellent resultsare attained using an EIES system.

During the course of formation of p-n type heterojunctions 34 inaccordance with the present invention, the controllers 60, 61 wereadjusted to insure relative elemental evaporation rates sufficient toproduce p-type films of 5 KΩ/□ to 800 KΩ/□ sheet resistivity for filmshaving thicknesses ranging from 2 μm to 3 μm. Typical deposition rateswere 2 Å/sec. for indium, 0.9 Å/sec. for copper, and 8-15 Å/sec. forselenium, which resulted in a CuInSe₂ deposition rate of 8 Å/sec. Sheetresistivity as a function of substrate position indicated excellentuniformity (±10% for low resistivities and a factor of 2-3 for highresistivities) was achieved. Reflection and transmission electrondifraction confirmed that the layers deposited were single phase,chalcopyrite CuInSe₂ and that grain sizes in excess of 10 μm werepresent in the low resistivity of p-type region 35a.

As previously indicated, one of the principal problems encountered inthe formation of thin-film, p-n-type heterojunction devices formed ofmaterials selected from the class of I-III-VI₂ chalcopyrite compoundshas involved the formation of growth nodules in the selenide layer 35.In each and every instance where the photocell exhibited the presence ofsuch growth nodules, the cells were found to have relatively low energyconversion efficiencies. Moreover, whereas high efficiency cells made inaccordance with the present invention exhibited improved photoresponsecharacteristics following post-deposition heat treatments, those cellshaving growth nodules present in the selenide layer tended to rapidlydegrade when exposed to subsequent heat treatments. It has been observedthat the conditions necessary for nodule formation are: (i) the selenidefilm must be of low resistivity (i.e., less than approximately 50 KΩ/□);(ii) the selenide film must be formed of p-type material; and (iii), theselenide film must be exposed to CdS. On the other hand, nodules havenot been detected in high resistivity p-type CuInSe₂ devices, nor inn-type CuInSe₂ devices. It is believed that excessive copper diffusioninto the CdS semiconductor layer serves to make the CdS layer arelatively high resistivity layer, thereby significantly reducing cellefficiency.

Referring to FIG. 7, there has been microphotographically illustrated at2000×magnification a conventional CuInSe₂ semiconductor wherein theselenide region 35b exhibits the presence of a plurality of undesiredcopper growth nodules 65 even when the region 35b has been exposed onlyto minimal amounts of CdS. Referring to FIG. 8, the same growth nodules65 are depicted in the junction 34 following application of the CdSlayer 36. It is believed that free copper in contact with the CdS,together with the large voids which have developed, explain why cellsformed with such nodules are of poor quality and further degrade withsubsequent heat treatment. In order to avoid copper growth noduleformation, the tendency in the prior art has, therefore, been to depositselenide film layers having relatively high surface resistivities whichhave been found not to exhibit nodule formations. With these structures,very high photocurrents have been achieved but, at quite low voltages.And, attempts to increase voltages by depositing low resistivity filmshave met with little success due to nodule formation.

However, when forming cells 30 in accordance with the features of thepresent invention wherein a relatively high resistivity n-type material(e.g., greater than 1×10⁶ Ω/□) is vacuum deposited over a moderately lowresistivity p-type region (e.g., 0.5-15.0 KΩ/□), the ensuinginterdiffusion (including the CdS layer) results in conversion of then-type region 35b (FIG. 5) to high resistivity p-type material. Bycarefully controlling the thickness of the low resistivity p-typeregion, the copper deposition rate, and substrate temperature,reproducible deposits without copper growth nodule formation have beenobtained, as best illustrated by reference to FIGS. 9 and 10.

Thus far we have, in the practice of the present invention, producedthin-film, p-n-type heterojunction solar cells exhibiting conversionefficiencies in excess of 9.0% and ranging up to on the order of 9.53%(percentages are here expressed as total area efficiencies as contrastedwith active area efficiencies--that is, a cell having a total areaefficiency of 9.53% and an exposed area of semiconductor material ofonly 95% exhibits an active area efficiency of approximately 10.0%). Forexample, a typical "as deposited" cell initially had an efficiency ofabout 5% with V_(oc) =325 mV and J_(sc) =31 mA/cm². Immediately after a20 minute 200° C. heat treatment in H₂ /Ar (probably with an airimpurity as evidenced by similar results using only air), the cellperformance improved to V_(oc) =375 mV, J_(sc) =34 mA/cm², n=7.83%, andF.F.=0.61. Thereafter, the cell efficiency showed continuous improvementwith time. Indeed, after twenty-five days it reached a stable value of8.72% as shown in FIG. 11 at 66. Improvement of efficiency is believedto have been mainly caused by a slowly increasing open circuit voltageand fill factor. The photovoltaic characteristics at the steady stateare shown at 70 in FIG. 12, which reflects:

    ______________________________________                                        V.sub.oc = 396 mV  J.sub.sc = 35 mA/cm.sup.2                                  n = 8.72%          F.F. = 0.64                                                ______________________________________                                    

The average total reflectance of the cell structure has been measuredand found to be approximately 14%, which is mainly from the front CdSsurface (n=2.2-2.3). A quarter-wavelength antireflection coating ofSiO_(x) (n≅1.55) was designed for a wavelength of 0.85 μm and evaporatedonto the high efficiency cell. The light I-V characteristic afterapplication of the SiO_(x) coating is shown in FIG. 12 by the curve 71.The short circuit current increased from 35 mA/cm² (without SiO_(x)coating) to 39 mA/cm², or more than a 10% improvement. The total areaperformances of the final cell are:

    ______________________________________                                        V.sub.oc = 396 mV  J.sub.sc = 39 mA/cm.sup.2                                  V.sub.mp = 293 mV  J.sub.mp = 33 mA/cm.sup.2                                  n = 9.53%          F.F. = 0.63                                                ______________________________________                                    

The foregoing measurements were made using an ELH lamp (a projector-typetungsten-halogen lamp) under simulated AM1 illumination. Excluding the5% grid shading area resulting from the test probe, the active areaefficiency was 10.15%. A similar cell from another substrate whichshowed an efficiency of 9.28% under the same illumination has beenmeasured under Seattle clear day sunlight (2:15 p.m., on June 19, 1980).The measured intensity was determined to be 92.5 mW/cm² utilizing astandard silicon cell. The cell characteristics are:

    ______________________________________                                        V.sub.oc = 380 mV   J.sub.sc = 35 mA/cm.sup.2                                 V.sub.mp = 280 mV   J.sub.mp = 30 mA/cm.sup.2                                 P.sub.m = 8.4 mW/cm.sup.2                                                     n = 9.1%            F.F. = 0.63                                               ______________________________________                                    

The efficiency under sunlight is less than 2% less than the measurementunder the ELH lamp.

Two representative high efficiency cells respectively havingefficiencies of 9.28% and 9.53% were then measured under a Xenon lampsolar simulator of NASA's Lewis Research Center. With the best availablereference cell (Cu₂ S/CdS cell with Kapton cover) whose spectralresponse resembles the response of cells made in accordance with thepresent invention, the measured photovoltaic performance was as follows:

    ______________________________________                                        9.28% Cell          9.53% Cell                                                ______________________________________                                        I.sub.sc = 39.3 mA  I.sub.sc = 38.8 mA                                        V.sub.oc = 391 mV   V.sub.oc = 404 mV                                         I.sub.max = 33.5 mA I.sub.max = 298 mV                                        V.sub.max = 286 mV                                                            P.sub.max = 9.58 mW P.sub.max = 9.89 mW                                       F.F. = 0.624        F.F. = 0.630                                              Eff. = 9.58%        Eff. = 9.89%                                              ______________________________________                                    

The foregoing devices exhibited spectral characteristics similar tothose previously reported by Kazmerski. See, e.g., Reference No. 8,supra. As shown in FIG. 13, the quantum efficiency as a function ofwavelength as measured at NASA's Lewis Research Center is fairly flat asindicated by curve 72 and its value is over 0.9, at least within themeasurement range from 0.6 μm≦λ≦1.0 μm.

The dark I-V characteristic of the high efficiency cell in a semilogplot is a straight line which gives the diode factor, A, 1.285 and thereverse saturation current, J_(o), 1.8×10⁻⁷ A/cm².

The response of high efficiency cells with an SiO_(x) coating as afunction of light intensity has been measured by a set of newly madeneutral density filters (various thicknesses of molybdenum on glass).These filters have a nearly flat transmittance over the 0.5 μm<λ<2.0 μmin contrast to previous Kodak gelatine filters which work only in thevisible range. The measured fill factor as a function of light intensityin terms of J_(L) /J_(o) is shown as a series of black dots 75 definingcurve 76 in FIG. 14. The intensity range is from 100 mW/cm² (J_(L)/J_(o) =1.94×10⁵) down to less than 10 mW/cm² (J_(L) /J_(o) =10⁴). Thesmooth curve 78 is the calculated fill factor as a function of J_(L)/J_(o) using the measured values of: R_(s) =1.2Ω; R_(p) =10⁵ Ω; A=1.285;J_(o) =1.8×10⁻⁷ A/cm² ; and, T=300°K--using the theory described by K.W. Mitchell, Evaluation Of The CdS/CdTe Heterojunction Solar Cell,GARLAND PUBLISHING, INC. (1979). The experimental and calculated valuesseem to agree very well, indicating that the fill factor is limited bythe series resistance. If the series resistance could be reduced to 0.5Ω(as indicated by the upper curve 76 in FIG. 14), the fill factor couldbe increased to 0.69 at the AM1 condition. And, if a high 0.69 fillfactor can be realized in the existing high efficiency cell (n=9.53%),the total area efficiency can be increased to 10.59%.

It should be noted that, in contrast to previous measurements whichshowed increasing efficiency by reducing light intensity, measurementsusing the new neutral density filters show that efficiency decreaseswith the decreasing light intensity.

It is further believed that additional increases in efficiency can bereadily attained by increasing the optical transparency of the contactgrid 38, 39. Thus, by reducing the number of electrode grid lines 38from ten lines per centimeter to five lines per centimeter, and bymoving the collector electrode to the edge of the cell (as indicated inFIG. 1)--in the high efficiency solar cells tested and reported onherein, such collector electrode was disposed in the center of the celland, consequently, measurements utilizing probes tended to shadow thecell and reduced the amount of light--it is expected that a 4% to 8%improvement in device efficiency will be attained.

Thus far, the present invention has been described in connection with atypical laboratory-type experimental system of the type shown in FIG. 6for forming small area, thin-film, p-n-type, heterojunction devicesreadily adaptable for formation by known low cost, large areafabrication techniques for applying film coatings to low cost, largearea substrates. As an interim step in converting such an experimentalsystem to a continuous in-line production system, a planetary-typedeposition fixture (not shown) might be employed wherein the substratesare mounted in planets which rotate about the planet axis and about thesystem axis. Such a conventional planetary system eliminates problems ofdeposition non-uniformities and enables use of parallel boats for thecopper and indium sources, or a conventional electron gun system (notshown) employing separate pockets for the copper and indium sources.

However, in its broader aspects, the present invention provides forformation of novel "graded-composition" heterojunctions 34 (FIG. 5)formed of materials selected from the class of I-III-VI₂ chalcopyritecompounds and CdS (or other suitable II-VI compounds) using novelprocess steps which readily permit of application of the process to acontinuous in-line system of the type generally indicateddiagrammatically at 80 in FIG. 15. In this type of system, substrates131 are continuously fed along a suitable conveyor system, indicateddiagrammatically at 81, through a substrate cleaning station 82. As thesubstrates exit the cleaning station 82, they enter a continuous in-linevacuum chamber 84 having differential vacuum pumps generally indicatedat 85 and sequentially presented process areas 88-92, with thesubstrates ultimately exiting from the vacuum chamber 84 at 94.

Process area 88 comprises a vacuum chamber preferably controlled attemperatures and pressure conditions suitable for application of amolybdenum or similar base contact material to the substrate byconventional sputtering or vacuum deposition techniques. In theexemplary system 80 of FIG. 15, the Mo contact is applied by using a Motarget 95 coupled to a suitable high voltage source 96 in a conventionalmanner well known to those skilled in the art.

Process area 89 is preferably maintained at a temperature on the orderof at least 350° and ranging to 450° by means of an adjustabletemperature control 98 and heating coil 99 so as to permit vacuumdeposition of CuInSe₂ on the substrates as they transit through vacuumchamber 84. A second adjustable heat control 100 and heating coil 101may be provided at the downstream end of zone 89 for raising thesubstrate temperature to about 450° C.±25° C., but less than 500° C.,after about 75% of the CuInSe₂ layer has been deposited. A suitable EIEScontroller or the like (not shown in FIG. 15) would be provided formonitoring and adjusting the copper/indium ratio in the mannerpreviously described. As previously indicated, the pressure in area 89of chamber 84 is preferably maintained at about 3-8×10⁻⁶ torr.

As the substrates successively pass through process areas 90, 91 and 92,the low resistivity CdS (or other suitable II-VI materials having bandgap energies greater than 1.5 ev) semiconductor layer 36, aluminumcontact grid 38, 39, and SiO_(x) layer 40 (Cf., FIG. 5) are sequentiallyapplied thereto. To this end, a temperature control 102 and heating coil104 are provided for establishing a controlled temperature level inprocess area 90 ranging from about 150° C. to about 250° C.; while asimilar adjustable control 105 and coil 106 are provided in process area92 for maintaining the temperature in a range of about 100° C. to about125° C. Aluminum is preferably evaporated in process area 91 at ambienttemperature and at a pressure less than 5×10⁻⁶ torr; while CdS ispreferably evaporated in process area 90 at a pressure of less than2×10⁻⁵ torr. Finally, SiO_(x) is deposited in process area 92 at adeposition rate and oxygen pressure adjusted to yield n≅1.55.

Thus, those persons skilled in the art will appreciate that there hasherein been provided a system which readily permits application ofmaterials selected from the class of I-III-VI₂ chalcopyrite compounds toa substrate to form a semiconductor layer having a composition gradientstherein, with the first semiconductor region applied to the substratebeing copper-enriched and having a relatively low resistivity--viz., onthe order of 0.5 to 15 KΩ/□--in the form of a p-type material; and,thereafter, a second region of the same elemental composition but, witha decreased copper/indium ratio--i.e., the material iscopper-deficient--is deposited on the first low resistivity region toform a relatively high resistivity (viz., on the order of greater than1×10⁶ Ω/□) transient n-type region. As a consequence of thisarrangement, when the CdS semiconductor film is vacuum deposited on the"composition-graded" chalcopyrite materials (which then define atransient p-n-type homojunction), copper growth nodules are precludedfrom forming and the transient n-type region of the chalcopyritesemiconductor gradually evolves to a high resistivity p-type regionthrough interdiffusion processes, thereby resulting in thin-filmheterojunction devices having energy conversion efficiencies whichclosely approximate 10%, or greater.

As previously indicated, the present invention finds particularlyadvantageous use in the formation of p-n-type heterojunctions for use insolar cells where the requisite band gap energy level for the p-typematerial falls in the range of 1-1.5 ev. However, since some n-typematerials in the class of I-III-VI₂ chalcopyrite compounds fall withinthat range--for example, AgInSe₂ having a band gap energy level of 1.24ev--it will be apparent that the present invention is not limited to theformation of p-n-type heterojunctions. Moreover, some of the I-III-VI₂chalcopyrite compounds such, for example, as CuInSe₂ can be grown asboth n-type and p-type crystals. This, therefore, permits the method ofthe present invention to also be used to form n-p-type heterojunctions.

For example, there has been diagrammatically illustrated in FIG. 16 atypical n-p-type heterojunction which, like the p-n-type heterojunctionpreviously described, is here preferably formed from CdS and CuInSe₂. Ashere shown, the exemplary device 110 is provided with a substrate 111preferably formed of glass, ITO (indium tin oxide) or similar lighttransmissive material so as to permit incident radiation to passtherethrough and into the photoactive region of the cell 110, asindicated by the arrow IR. A suitable base contact 112 is applieddirectly to the substrate 111 and, since such contact must be lighttransparent, it is preferably formed in a grid-like pattern or, ofindium or the like. Thereafter, a first semiconductor layer 115 of, forexample, CdS is applied to the contact 112, and a composition gradedlayer of photoactive material 116 such, for example, as CuInSe₂, isapplied to layer 115 to form the desired n-p heterojunction 114. Inkeeping with the invention, the n-type layer of CdS (or other suitableII-VI type material having a band gap energy greater than 1.5 ev)includes a first indium-doped region 115a in contact with the basecontact 112, and a second superimposed region 115b of pure CdS.Similarly, the layer 116 comprises composition graded regions ofI-III-VI₂ type ternary material such, for example, as CuInSe₂, whereinthe lower region 116a comprises a high resistivity transient n-typeregion of copper-deficient CuInSe₂, and the upper region 116b comprisesa low resistivity p-type region of copper-enriched CuInSe₂. Finally, anupper contact 118 is formed on region 116b to complete the transducer110.

In general, the various layers of the cell 110 indicated by way ofexample in FIG. 16 can be applied in a manner similar to that previouslydescribed; except, that the temperature level for applying thecomposition graded layer 116 of photoactive material is preferablymaintained at less than 250° since this layer is deposited on the CdSlayer 115. Additionally, the substrate 111 and base contact must belight transparent as indicated above. This type of cell has oneadvantage over the p-n junction previously described in that the glasssubstrate 111 or the like serves to protect the cell; whereas cells suchas that shown in FIGS. 2 and 5 are preferably, but not necessarily,provided with some type of additional protection such, for example, asglass encapsulation or the like.

Thus, it will be evident to those skilled in the art that the presentinvention permits the formation of both p-n-type heterojunction andn-p-type heterojunctions which are formed by a process in which,preferably, a I-III-VI₂ type photoactive material is deposited bysimultaneous elemental evaporation to initially form a firstsemiconductor layer defining a transient homojunction which subsequentlyevolves into a composition graded photoactive semiconductor layer havinga low resistivity region and a high resistivity region with the lattersandwiched between the low resistivity region and a second semiconductorlayer of II-VI type semiconductor material. Consequently, since thethin-film heterojunction may be formed as either a p-n-type device or ann-p-type device, such heterojunctions are referred to herein and incertain of the appended claims as an "A-B-type" heterojunction whereinthe "A" and "B" layers of the heterojunction formed are selected fromone of the following four combinations of A/B semiconductor materials:

    ______________________________________                                        A                and    B                                                     ______________________________________                                        (i)  a p-type ternary material                                                                     and    an n-type material;                               (ii) an n-type ternary                                                                             and    a p-type material;                                     material                                                                 (iii)                                                                              an n-type material                                                                            and    a p-type ternary material;                        (iv) a p-type material                                                                             and    an n-type ternary material.                       ______________________________________                                    

It will further be understood by those skilled in the art that theinvention is not limited to ternary type materials and that othermaterials may also be suitable. For example, it is believed that aquaternary material such as CuIn_(1-x) Ga_(x) Se₂ or CuIn_(1-x) Ga_(x)S₂ could be used. Thus, the term "ternary" as used herein and in theappended claims is not used in a limiting sense, but only in the sensethat the compound from which the photoactive region of the cell isformed must have at least three constituent elements including at leasttwo elements selected from the class of I-III elements such, forexample, as copper and indium, and at least one element selected from aclass VI material such as Se or S. Similarly, while excellent resultshave been obtained utilizing CdS as the n-type material--a materialhaving a band gap energy of 2.4 ev--other materials could be used such,for example, as Cd_(1-x) Zn_(x) S having a band gap energy greater than2.4 or CdS_(1-x) Se_(x) having a band gap energy greater than 2 ev.Thus, it is evident that the n-type material is preferably selected fromthe class of II-VI elements having suitable wide gap energies on theorder of greater than 1.5 ev.

It is to be further understood that the phrase "simultaneous elementalevaporation" as used herein and in the appended claims means thesimultaneous evaporation of the three constituent elements from varioustypes of sources such, for example, as: (i) a compound ternary source ofCuInSe₂ ; or (ii), two separate sources such as Cu₂ Se and In₃ Se₃ ; or(iii), three separate sources such as Cu₂ Se, In and Se; etc. Otherpossibilities also exist as will be recognized by those skilled in theart.

Finally, those skilled in the art will appreciate that the referencesherein and in the appended claims to "composition graded" regions ofphotovoltaic semiconductor materials is intended to embrace thoseregions of a transient homojunction resulting from alteration of theratio of the metallic elements selected from the class of I-IIImaterials in a ternary or quaternary compound or the like. For example,where one desires to form a p-type region, such result can be achievedby providing an evaporant stream that either contains an excess of atype I element or a deficiency of a type III element; and, where onewishes to obtain a transient n-type material, such result can beobtained by evaporating a stream that is deficient in a type I elementor enriched with a type III element.

What is claimed is:
 1. In a method of forming a photovoltaiclight-to-electrical energy transducer of the type including a thin-filmA-B-type heterojunction where "A" and "B" are selected from the group ofsemiconductor materials consisting of:

    ______________________________________                                        A                  and     B                                                  ______________________________________                                        (i)   a p-type ternary material                                                                      and     an n-type material;                            (ii)  an n-type ternary material                                                                     and     a p-type material;                             (iii) an n-type material                                                                             and     a p-type ternary                                                              material;                                      (iv)  a p-type material                                                                              and     an n-type ternary                                                             material;                                      ______________________________________                                    

and wherein the transducer includes a substrate, a first contactdeposited on the substrate, a first semiconductor layer formed of A-typematerial deposited on the first contact, a second semiconductor layerformed of B-type material deposited on the first semiconductor layer anddefining therewith the thin-film A-B-type heterojunction, and a secondcontact deposited on the second semiconductor layer, the improvementcomprising a method wherein: (a) the one of the first and secondsemiconductor layers formed of a ternary semiconductor material isformed by simultaneous elemental evaporation of the ternarysemiconductor material to form a semiconductor layer having twocomposition graded regions sequentially formed one upon the other withone region having a first preselected ratio of two of the elements inthe ternary semiconductor material so as to form a low resistivitysemiconductor region and the other of the regions having a differentpreselected ratio of the same two elements so as to form a highresistivity transient semiconduction region and with the two regionsdefining a transient homojunction; and (b) the other of the first andsecond semiconductor layers is formed by deposition of a semiconductormaterial in face-to-face contact with respect to the high resistivitytransient semiconductor region of the transient homojunction so as topermit the high resistivity transient semiconductor region to evolvethrough elemental interdiffusion into a region of relatively highresistivity semiconductor material of the same type as the lowresistivity region formed in step (a) to thereby form a thin-film A-Btype heterojunction photovoltaic light-to-electrical energy transducer.2. The method of forming a photovoltaic light-to-electrical energytransducer including a thin-film A-B-type heterojunction as set forth inclaim 1 wherein the A-B-type heterojunction is a p-n-type heterojunctionand the one semiconductor layer defining the transient homojunctiondefines a transient p-n-type homojunction having a region of lowresistivity p-type material and a region of high resistivity transientn-type material, and the other semiconductor layer is formed of n-typesemiconductor material deposited in face-to-face contact with the regionof high resistivity transient n-type material.
 3. The method of forminga photovoltaic light-to-electrical energy transducer including athin-film A-B-type heterojunction as set forth in claim 1 wherein theA-B-type heterojunction is an n-p-type heterojunction and the onesemiconductor layer defining the transient homojunction defines atransient n-p type homojunction having a region of high resistivitytransient n-type material and a region of low resistivity p-typematerial, and the other semiconductor layer is formed of n-typesemiconductor material deposited in face-to-face contact with the regionof high resistivity transient n-type material.
 4. The method of forminga photovoltaic light-to-electrical energy transducer including athin-film A-B-type heterojunction as set forth in claim 1 wherein theternary material is selected from the group of I-III-VI₂ chalcopyritecompounds.
 5. The method of forming a photovoltaic light-to-electricalenergy transducer including a thin-film A-B-type heterojunction as setforth in claim 1 wherein the ternary material is selected from the groupof I-III-VI₂ chalcopyrite compounds and the other of the first andsecond semiconductor layers is formed of materials selected from thegroup of II-VI elements.
 6. The method of forming a photovoltaiclight-to electrical energy transducer including a thin-film A-B-typeheterojunction as set forth in claims 4 or 5 wherein the ternarymaterial has a band gap energy in the range of 1-1.5 ev.
 7. The methodof forming a photovoltaic light-to-electrical energy transducerincluding a thin-film A-B-type heterojunction as set forth in claims 4or 5 wherein the ternary material has a band gap energy in the range of1-1.5 ev and the other of the first and second semiconductor layers isformed of materials having a band gap energy greater than 1.5 ev.
 8. Themethod of forming a photovoltaic light-to-electrical energy transducerincluding a thin-film A-B-type heterojunction as set forth in claims 1,2 or 3 wherein the ternary semiconductor material is CuInSe₂.
 9. Themethod of forming a photovoltaic light-to-electrical energy transducerincluding a thin-film A-B-type heterojunction as set forth in claims 1,2 or 3 wherein the other of the first and second semiconductor layershas a band gap energy greater than 1.5 ev.
 10. The method of forming aphotovoltaic light-to-electrical energy transducer including a thin-filmA-B-type heterojunction as set forth in claims 1, 2 or 3 wherein theother of the first and second semiconductor layers is selected from thegroup consisting ofCdS; Cd_(1-x) Zn_(x) S; and, CdS_(1-x) Se_(x). 11.The method of forming a photovoltaic light-to-electrical energytransducer including a thin-film A-B-type heterojunction as set forth inclaims 1, 2 or 3 wherein the ternary semiconductor material is CuInSe₂and the other of the first and second semiconductor layers is selectedfrom the group consisting of:CdS; Cd_(1-x) Zn_(x) S; and, CdS_(1-x)Se_(x).
 12. The method of forming a photovoltaic light-to-electricalenergy transducer including a thin-film A-B-type heterojunction as setforth in claims 1, 2, or 3 wherein the ternary semiconductor material isCuInSe₂ and the other of the first and second semiconductor layers isCdS.
 13. The method of forming a photovoltaic light-to-electrical energytransducer including a thin-film A-B-type heterojunction as set forth inclaims 1, 2 or 3 wherein the ternary semiconductor material is aI-III-VI₂ chalcopyrite compound and the ratio of the I-III elements isadjusted during one portion only of simultaneous elemental evaporationof the ternary compound so as to form one of the low resistivity andtransient high resistivity regions of the transient homojunction, andadjusted to a different I-III ratio during the remaining portion ofsimultaneous elemental evaporation of the ternary compound so as to formthe other of the low resistivity and the transient high resistivityregions of the transient homojunction.
 14. The method of forming aphotovoltaic light-to-electrical energy transducer including thethin-film A-B-type heterojunction as set forth in claims 1 or 2 whereinthe ternary semiconductor layer is CuInSe₂ and the copper/indium ratiois initially adjusted to form a slightly copper-enriched region duringsimultaneous elemental evaporation of the CuInSe₂ to form the lowresistivity region of the transducer of p-type material, and isreadjusted to form a slightly copper-deficient region duringsimultaneous elemental evaporation of the CuInSe₂ to form the highresistivity transient semiconductor region of the transducer oftransient n-type material.
 15. The product produced by the method setforth in claims 1, 2, 3, 4 or
 5. 16. In a photovoltaiclight-to-electrical energy transducer of the type including a thin-filmA-B-type heterojunction where "A" and "B" are selected from the group ofsemiconductor materials consisting of:

    ______________________________________                                        A                  and     B                                                  ______________________________________                                        (i)   a p-type ternary material                                                                      and     an n-type material;                            (ii)  an n-type ternary material                                                                     and     a p-type material;                             (iii) an n-type material                                                                             and     a p-type ternary                                                              material;                                      (iv)  a p-type material                                                                              and     an n-type ternary                                                             material;                                      ______________________________________                                    

and wherein the transducer includes a substrate, a first contactdeposited on said substrate, a first semiconductor layer formed ofA-type material deposited on said first contact, a second semiconductorlayer formed of B-type material deposited on said first semiconductorlayer and defining therewith the thin-film A-B-type heterojunction, anda second contact deposited on said second semiconductor layer, theimprovement comprising: (a) the one of said first and secondsemiconductor layers formed of a ternary semiconductor materialcomprising a semiconductor layer having been formed with two compositiongraded regions with one region superimposed on the other and with oneregion having a first preselected ratio of two of the elements in saidternary semiconductor material so as to form a low resistivitysemiconductor region and the other of said regions having a differentpreselected ratio of the same two elements and having been formed as ahigh resistivity transient semiconductor region and with said tworegions having been formed as a transient homojunction; and, (b) theother of said first and second semiconductor layers having been formedin face-to-face contact with said high resistivity transientsemiconductor region of said transient homojunction so as to permit saidhigh resistivity transient semiconductor region to evolve throughelemental interdiffusion into a region of relatively high resistivitysemiconductor material of the same type as said low resistivity regionto thereby form a thin-film A-B-type heterojunction photovoltaiclight-to-electrical energy transducer.
 17. The photovoltaiclight-to-electrical energy transducer set forth in claim 16 andincluding a thin-film A-B-type heterojunction wherein said A-B-typeheterojunction is a p-n-type heterojunction and said one semiconductorlayer defining said transient homojunction having been formed as atransient p-n-type homojunction having a region of low resistivityp-type material and a region of high resistivity transient n-typematerial, and said other semiconductor layer is formed of n-typesemiconductor material deposited in face-to-face contact with saidregion of high resistivity transient n-type material.
 18. Thephotovoltaic light-to-electrical energy transducer set forth in claim 16and including a thin-film A-B-type heterojunction wherein said A-B-typeheterojunction is an n-p-type heterojunction and said one semiconductorlayer defining said transient homojunction having been formed as atransient n-p type homojunction having a region of high resistivitytransient n-type material and a region of low resistivity p-typematerial, and said other semiconductor layer is formed of n-typesemiconductor material deposited in face-to-face contact with saidregion of high resistivity transient n-type material.
 19. Thephotovoltaic light-to-electrical energy transducer as set forth in claim16 and including a thin-film A-B-type heterojunction wherein saidternary material is selected from the group of I-III-VI₂ chalcopyritecompounds.
 20. The photovoltaic light-to-electrical energy transducer asset forth in claim 16 and including a thin-film A-B-type heterojunctionwherein said ternary material is selected from the group of I-III-VI₂chalcopyrite compounds and said other of said first and secondsemiconductor layers is formed of materials selected from the group ofII-VI elements.
 21. The photovoltaic light-to-electrical energytransducer as set forth in claims 19 or 20 and including a thin-filmA-B-type heterojunction wherein said ternary material has a band gapenergy in the range of 1-1.5 ev.
 22. The photovoltaiclight-to-electrical energy transducer as set forth in claims 19 or 20and including a thin film A-B-type heterojunction wherein said ternarymaterial has a band gap energy in the range of 1-1.5 ev and said otherof said first and second semiconductor layers is formed of materialshaving a band gap energy greater than 1.5 ev.
 23. The photovoltaiclight-to-electrical energy transducer as set forth in claims 16, 17 or18 and including a thin-film A-B-type heterojunction wherein saidternary semiconductor material is CuInSe₂.
 24. The photovoltaiclight-to-electrical energy transducer as set forth in claims 16, 17 or18 and including a thin-film A-B-type heterojunction wherein said otherof said first and second semiconductor layers has a band gap energygreater than 1.5 ev.
 25. The photovoltaic light-to-electrical energytransducer as set forth in claims 16, 17 or 18 and including a thin-filmA-B-type heterojunction wherein said other of said first and secondsemiconductor layers is selected from the group consisting of:CdS;Cd_(1-x) Zn_(x) S; and, CdS_(1-x) Se_(x).
 26. The photovoltaiclight-to-electrical energy transducer as set forth in claims 16, 17 or18 and including a thin-film A-B-type heterojunction wherein saidternary semiconductor material is CuInSe₂ and said other of said firstand second semiconductor layers is selected from the group consistingof:CdS; Cd_(1--x) Zn_(x) S; and, CdS_(1--x) Se_(x).
 27. The photovoltaiclight-to-electrical energy transducer as set forth in claims 16, 17 or18 and including a thin-film A-B-type heterojunction wherein saidternary semiconductor material is CuInSe₂ and said other of said firstand second semiconductor layers in CdS.
 28. The method of forming ap-n-type heterojunction photovoltaic device comprising the steps of:(a)depositing a first region of relatively low resistivity p-type materialon a metallized substrate; (b) depositing a second region of relativelyhigh resistivity transient n-type material formed of the same elementalconstituents as the relatively low resistivity p-type material depositedin step (a) with such transient n-type material being deposited on thefirst region of p-type material and defining therewith a transientp-n-type homojunction; and, (c) depositing a film of low resistivityn-type semiconductor material on the transient p-n-type homojunctionformed in steps (a) and (b), whereupon interdiffusion of the constituentelements of the materials employed in steps (a), (b) and (c) between thep-type matrial and the transient n-type material, and between thetransient n-type material and the n-type semiconductor material, causesthe transient n-type material to evolve into relatively high resistivityp-type material so as to form a thin-film heterojunction essentiallydevoid of growth nodules and permitting a photovoltaic responsecharacteristic of energy transducers capable of exhibiting relativelyhigh conversion efficiencies at least approximating 10.0%.
 29. Themethod of claim 28 wherein the first and second regions of p-type andtransient n-type material, respectively, comprise a ternarysemiconductor material formed by simultaneous elemental evaporation. 30.The method of claim 29 wherein the ternary semiconductor material is achalcopyrite compound.
 31. The method of claim 29 wherein the ternarysemiconductor material is a material selected from the class ofI-III-VI₂ chalcopyrite compounds.
 32. The method of claim 29 wherein theternary semiconductor material is CuInSe₂.
 33. The method of claim 32wherein the copper/indium ratio in the ternary semiconductor material isinitially adjusted to form a slightly copper-enriched region duringsimultaneous elemental evaporation thereof to form the first region ofrelatively low resistivity p-type material in step (a) and is readjustedduring such simultaneous elemental evaporation to form a slightlycopper-deficient ternary compound during evaporation of the secondregion of relatively high resistivity transient n-type material formedin step (b).
 34. The method of claim 32 wherein the copper/indium ratiois altered upon formation of at least about 50% of the total desiredthickness of the transient p-n-type homojunction but prior to formationof about 66.7% of the total desired thickness of the transient p-n-typehomojunction.
 35. The method of claim 32 wherein the film of lowresistivity n-type semiconductor material is formed of n-type materialhaving a band gap energy greater than 1.5 ev.
 36. The method of claim 35wherein the n-type low resistivity semiconductor material is a II-VImaterial.
 37. The method of claim 36 wherein the II-VI material isselected from the group consisting of:CdS; Cd_(1--x) Zn_(x) S; and,CdS_(1--x) Se_(x).
 38. The method of claim 36 wherein the II-VI materialis CdS.
 39. The method of claim 28 wherein the film of low-resistivityn-type semiconductor material comprises a first region of relativelypure CdS and a second superimposed region of indium-doped CdS.
 40. Themethod of claim 28 wherein the first and second regions of semiconductormaterial formed in steps (a) and (b) are formed at a temperature in therange of 350° C. to 500° C.
 41. The method of claim 28 wherein the firstregion of semiconductor material formed in step (a) and a portion of thesecond region of semiconductor material formed in step (b) are formed ata temperature in the range of 350° C. to a temperature less than 450°C., and the remainder of the second region of semiconductor materialformed in step (b) is formed at a temperature in the range of about 450°to 500° C.
 42. The method of claim 28 wherein the first region ofsemiconductor material formed in step (a) and a portion of the secondregion of semiconductor material formed in step (b) are formed at atemperature on the order of 350° C., and the remainder of the secondregion of semiconductor material formed in step (b) is formed at atemperature on the order of 450° C.±approximately 25° C.
 43. The methodof claim 28 wherein the film of low resistivity n-type semiconductormaterial formed in step (c) is formed at a temperature in the range of150° C. to 200° C.
 44. The method of claims 40, 41 or 42 wherein thefilm of low resistivity n-type semiconductor material formed in step (c)is formed at a temperature in the range of 150° C. to 200° C.
 45. Themethod of claims 28, 30, 31, 32, 40, 41 or 42 wherein the first andsecond regions of semiconductor material formed in steps (a) and (b) areformed in an atmosphere maintained at 3-8×10⁻⁶ torr.
 46. The method ofclaims 28, 29, 30, 31 or 32 wherein a grid-like contact is formed on thefilm of low resistivity n-type semiconductor material deposited in step(c) and an antireflective coating is formed on the grid-like contact andthe exposed surface of the film of low resistivity n-type semiconductormaterial.
 47. The method of claims 28, 29, 30, 31 or 32 wherein agrid-like contact is formed on the film of low resistivity n-typesemiconductor material deposited in step (c) and anantireflectivecoating formed of SiO_(x) is formed on the grid-likecontact and the exposed surface of the film of low resistivity n-typesemiconductor material.
 48. The method of claims 28, 29, 30, 31 or 32wherein a grid-like contact is formed on the film of low resistivityn-type semiconductor material deposited in step (c) and anantireflective coating formed of SiO_(x) wherein "x" is on the order of"1.8" is formed on the grid-like contact and the exposed surface of thefilm of low resistivity n-type semiconductor material.
 49. The productproduced by the method set forth in claims 28, 29, 30, 31, 32, 40, 41,42 or
 43. 50. In a method of forming a photovoltaic light-to-electricalenergy transducer of the type comprising a thin-film p-n-typeheterojunction including a metallized substrate, a first semiconductorlayer formed of p-type semiconductor material and deposited on themetallized substrate, a second semiconductor layer formed of lowresistivity n-type semiconductor material formed on the firstsemiconductor layer, and a grid-like upper contact formed on the secondsemiconductor layer, the improvement comprising a method wherein:(a) thefirst semiconductor layer of a ternary semiconductor material is formedby simultaneous elemental evaporation to form a first region of lowresistivity p-type semiconductor material on the metallized substrate;and, (b) while the ternary material is undergoing simultaneous elementalevaporation the ratio of two of the elemental constituents beingevaporated is adjusted so as to form a second region of relatively highresistivity transient n-type semiconductor material on the first regionof low resistivity p-type material, thereby forming a transient p-n-typehomojunction on the metallized substrate; and, wherein upon formation ofthe second semiconductor layer of n-type material on the transient p-nhomojunction, the second region of relatively high resistivity transientn-type semiconductor material is sandwiched between the first region oflow resistivity p-type material and the second semiconductor layer oflow resistivity n-type material so as to permit the transient n-typeregion to evolve through elemental interdiffusion into a region ofrelatively high resistivity p-type material to thereby form a thin-filmp-n-type heterojunction photovoltaic light-to electrical energytransducer.
 51. The method of claim 50 wherein the first and secondregions of p-type and transient n-type material are formed bysimultaneous elemental evaporation of a ternary semiconductor material.52. The method of claim 51 wherein the ternary semiconductor material isa chalcopyrite compound.
 53. The method of claim 51 wherein the ternarysemiconductor material is a material selected from the class ofI-III-VI₂ chalcopyrite compounds.
 54. The method of claim 51 wherein theternary semiconductor material is CuInSe₂.
 55. The method of claim 54wherein the copper/indium ratio in the ternary semiconductor material isinitially adjusted to form a slightly copper-enriched region duringsimultaneous elemental evaporation thereof to form the first region ofrelatively low resistivity p-type material in step (a) and is readjustedduring such simultaneous elemental evaporation to form a slightlycopper-deficient ternary compound during evaporation of the secondregion of relatively high resistivity transient n-type material formedin step (b).
 56. The method of claim 54 wherein the copper/indium ratiois readjusted upon formation of at least about 50% of the total desiredthickness of the transient p-n-type homojunction but prior to formationof about 66.7% of the total desired thickness of the transient p-n-typehomojunction.
 57. The method of claim 54 wherein the secondsemiconductor layer of low resistivity n-type semiconductor material inCdS.
 58. The method of claim 57 wherein the layer of low-resistivityn-type semiconductor material comprises a first region of relativelypure CdS and a second superimposed region of indium-doped CdS.
 59. Themethod of claim 57 wherein the first and second regions of semiconductormaterial formed in steps (a) and (b) are formed at a temperature in therange of 350° C. to 500° C.
 60. The method of claim 57 wherein the firstregion of semiconductor material formed in step (a) and a portion of thesecond region of semiconductor material formed in step (b) are formed ata temperature in the range of 350° C. to a temperature less than 450°C., and the remainder of the second region of semiconductor materialformed in step (b) is formed at a temperature in the range of about 450°to 500° C.
 61. The method of claim 57 wherein the first region ofsemiconductor material formed in step (a) and a portion of the secondregion of semiconductor material formed in step (b) are formed at atemperature on the order of 350° C., and the remainder of the secondregion of semiconductor material formed in step (b) is formed at atemperature on the order of 450° C.±approximately 25° C.
 62. The methodof claim 57 wherein the layer of low resistivity n-type semiconductormaterial is formed at a temperature in the range of 150° C. to 200° C.63. The method of claims 59, 60 or 61 wherein the layer of lowresistivity n-type semiconductor material is formed at a temperature inthe range of 150° C. to 200° C.
 64. The method of claims 50, 54, 55 or57 wherein the first and second regions of semiconductor material formedin steps (a) and (b) are formed in an atmosphere maintained at 3-8×10⁻⁶torr.
 65. The method of claims 50, 54, 55 or 57 wherein anantireflective coating is formed on the grid-like contact and theexposed surface of the second semiconductor layer.
 66. The method ofclaims 50, 54, 55 or 57 wherein an antireflective coating formed ofSiO_(x) is formed on the grid-like contact and the exposed surface ofthe second semiconductor layer.
 67. The method of claims 50, 54, 55 or57 wherein an antireflective coating formed of SiO_(x) wherein "x" is onthe order of "1.8" is formed on the grid-like contact and the exposedsurface of the semiconductor layer.
 68. The product produced by themethod set forth in claims 50, 54, 55 or
 57. 69. A p-n-typeheterojunction photovoltaic device comprising, in combination: ametallized substrate; a first relatively thin-film region of relativelylow resistivity p-type material adhered to said metallized substrate; asecond relatively thin-film region formed of the same elementalconstituents as said relatively low resistivity p-type material andhaving been formed as a relatively high resistivity transient n-typematerial region with said relatively low resistivity p-type material andsaid relatively high resistivity transient n-type material region havingbeen formed as a composite transient p-n-type homojunction semiconductorlayer; and, a relatively thin film of low resistivity n-typesemiconductor material having been deposited on said transient p-nhomojunction whereupon interdiffusion of the constituent elements of thematerials defining said p-type region, said transient n-type region andsaid n-type semiconductor material between the p-type material and thetransient n-type material, and between the transient n-type material andthe n-type semiconductor material causes the transient n-type materialto evolve into relatively high resistivity p-type material so as to forma thin-film heterojunction essentially devoid of growth nodules andpermitting a photovoltaic response characteristic of energy transducerscapable of exhibiting conversion efficiencies at least approximating10.0%.
 70. The device of claim 69 wherein said first and second regionsof p-type and transient n-type material comprise a ternary semiconductormaterial.
 71. The device of claim 70 wherein said ternary semiconductormaterial is a chalcopyrite compound.
 72. The device of claim 70 whereinsaid ternary semiconductor material comprises a material selected fromthe class of I-III-VI₂ chalcopyrite compounds.
 73. The device of claim70 wherein said ternary semiconductor material is CuInSe₂.
 74. Thedevice of claim 73 wherein the copper/indium ratio in said first regionof said ternary semiconductor material is such as to form a slightlycopper-enriched first region of relatively low resistivity p-typematerial and the copper/indium ratio in said second region of saidternary semiconductor material is such as to form a slightlycopper-deficient second region of relatively high resistivity transientn-type material.
 75. The device of claim 74 wherein said first region ofp-type material comprises between about 50% and about 66.7% of the totaldesired thickness of said transient p-n-type homojunction.
 76. Thedevice of claim 74 wherein said film of low resistivity n-typesemiconductor material is CdS.
 77. The device of claim 74 wherein saidfilm of low-resistivity n-type semiconductor material comprises a firstregion of relatively pure CdS and a second superimposed region ofindium-doped CdS.
 78. The device of claim 69 wherein a grid-like contactis formed on the surface of said thin film of low resistivity n-typesemiconductor material.
 79. The device of claim 78 wherein anantireflective coating is formed on said grid-like contact and on theexposed surface of said thin film of low resistivity n-typesemiconductor material.
 80. The device of claim 79 wherein saidantireflective coating comprises SiO_(x) wherein "x" is on the order of"1.8".
 81. In a photovoltaic light-to-electrical energy transducer ofthe type comprising a thin-film p-n-type heterojunction including ametalized substrate, a first semiconductor layer formed of p-typesemiconductor material deposited on said metallized substrate, a secondsemiconductor layer formed of low resistivity n-type semiconductormaterial formed on said first semiconductor layer, and a grid-like uppercontact formed on said second semiconductor layer, the improvementwherein:said first semiconductor layer includes a first region of lowresistivity, p-type semiconductor material formed on said metallizedsubstrate; and a second region having been formed as a relatively highresistivity transient n-type semiconductor material region formed onsaid first region of p-type material with said first and second regionshaving been formed as a transient p-n-type homojunction formed on saidmetallized substrate with said transient n-type semiconductor regionsandwiched between said low resistivity region of p-type semiconductormaterial and said second semiconductor layer formed of low resistivityn-type material so as to permit said transient n-type region to evolvethrough elemental interdiffusion into a region of relatively highresistivity p-type material so as to form a thin-film, p-n-typeheterojunction photovoltaic light-to-electrical energy transducer. 82.The transducer of claim 81 wherein said first and second regions ofp-type and transient n-type material comprise a ternary semiconductormaterial.
 83. The transducer of claim 82 wherein said ternarysemiconductor material is a chalcopyrite compound.
 84. The transducer ofclaim 82 wherein said ternary semiconductor material comprises amaterial selected from the class of I-III-VI₂ chalcopyrite compounds.85. The transducer of claim 82 wherein said ternary semiconductormaterial is CuInSe₂.
 86. The transducer of claim 85 wherein thecopper/indium ratio in said first region of said ternary semiconductormaterial is such as to form a slightly copper-enriched first region ofrelatively low resistivity p-type material and the copper/indium ratioin said second region of said ternary semiconductor material is such asto form a slightly copper-deficient second region of relatively highresistivity transient n-type material.
 87. The transducer of claim 86wherein said first region of p-type material comprises between about 50%and about 66.7% of the total desired thickness of said transientp-n-type homojunction.
 88. The transducer of claim 86 wherein saidsecond semiconductor layer formed of low resistivity n-typesemiconductor material is CdS.
 89. The transducer of claim 86 whereinsaid second semiconductor layer formed of low resistivity n-typesemiconductor material comprises a first region of relatively pure CdSand a second superimposed region of indium-doped CdS.
 90. The transducerof claim 81 wherein said grid-like upper contact formed on the surfaceof said second semiconductor layer is aluminum.
 91. The transducer ofclaim 90 wherein an anti-reflective coating is formed on said grid-likeconact and on the exposed surface of said second semiconductor layer.92. The transducer of claim 91 wherein said antireflective coatingcomprises SiO_(x) wherein "x" is on the order of "1.8".
 93. In a methodof forming a photovoltaic light-to-electrical energy transducer of thetype including a thin-film A-B-type heterojunction where "A" and "B" areselected from the group of semiconductor materials consisting of

    ______________________________________                                        A                  and     B                                                  ______________________________________                                        (i)   a p-type ternary material                                                                      and     an n-type material;                            (ii)  an n-type ternary material                                                                     and     a p-type material;                             (iii) an n-type material                                                                             and     a p-type ternary                                                              material;                                      (iv)  a p-type material                                                                              and     an n-type ternary                                                             material;                                      ______________________________________                                    

and wherein the transducer includes a substrate, a first contactdeposited on the substrate, a first semiconductor layer formed of A-typematerial deposited on the first contact, a second semiconductor layerformed of B-type material deposited on the first semiconductor layer anddefining therewith the thin-film A-B-type heterojunction, and a secondcontact deposited on the second semiconductor layer, the improvementcomprising a method wherein: (a) the one of the first and secondsemiconductor layers formed of a ternary semiconductor material isformed by simultaneous elemental evaporation of the ternarysemiconductor material to form a semiconductor layer having twocomposition graded regions sequentially formed one upon the other withone region having a first preselected ratio of two of the elements inthe ternary semiconductor material so as to form a low resistivitysemiconductor region and the other of the regions having a differentpreselected ratio of the same two elements so as to form a highresistivity transient semiconductor region and with the two regionsdefining a transient homojunction; (b) the other of the first and secondsemiconductor layers is formed by deposition of a semiconductor materialin face-to-face contact with respect to the high resistivity transientsemiconductor region of the transient homojunction; and, (c) the energytransducer formed is heated subsequent to steps (a) and (b); to therebyform a transducer wherein the high resistivity transient semiconductorregion formed in step (a) is permitted to evolve through elementalinterdiffusion into a region of relatively high resistivitysemiconductor material of the same type as the low resistivity regionformed in step (a) so as to form a thin-film A-B-type heterojunctionphotovoltaic light-to-electrical energy transducer.
 94. The method offorming a photovoltaic light-to-electrical energy transducer including athin-film A-B-type heterojunction as set forth in claim 93 wherein thetransducer formed is heated during step (c) in the presence of air. 95.The method of forming a photovoltaic light-to-electrical energytransducer including a thin-film A-B-type heterojunction as set forth inclaim 93 wherein the transducer formed is heated during step (c) in thepresence of H₂ /Ar and air.
 96. The method of forming a photovoltaiclight-to-electrical energy transducer including a thin-film A-B-typeheterojunction as set forth in claims 93, 94, or 95 wherein thetransducer formed is heated during step (c) at a temperature on theorder of 200° C.
 97. The method of forming a photovoltaiclight-to-electrical energy transducer including a thin-film A-B-typeheterojunction as set forth in claims 93, 94 or 95 wherein thetransducer formed is heated during step (c) at a temperature on theorder of 200° C. for a period on the order of 20 minutes.
 98. The methodof forming a photovoltaic light-to-electrical energy transducerincluding a thin-film A-B-type heterojunction as set forth in claims 93,94 or 95 wherein the A-B-type heterojunction is a p-n-typeheterojunction and the one semiconductor layer defining the transienthomojunction defines a transient p-n-type homojunction having a regionof low resistivity p-type material and a region of high resistivitytransient n-type material, and the other semiconductor layer is formedof n-type semiconductor material deposited in face-to-face contact withthe region of high resistivity transient n-type material.
 99. The methodof forming a photovoltaic light-to-electrical energy transducerincluding a thin-film A-B-type heterojunction as set forth in claims 93,94 or 95 wherein the A-B-type heterojunction is an n-p-typeheterojunction and the one semiconductor layer defining the transienthomojunction defines a transient n-p-type homojunction having a regionof high resistivity transient n-type material and a region of lowresistivity p-type material, and the other semiconductor layer is formedon n-type semiconductor material deposited in face-to-face contact withthe region of high resistivity transient n-type material.
 100. Themethod of forming a photovoltaic light-to-electrical energy transducerincluding a thin-film A-B-type heterojunction as set forth in claims 93,94 or 95 wherein the ternary material is selected from the group ofI-III-VI₂ chalcopyrite compounds.
 101. The method of forming aphotovoltaic light-to-electrical energy transducer including a thin-filmA-B-type heterojunction as set forth in claims 93, 94 or 95 wherein theternary material is selected from the group of I-III-VI₂ chalcopyritecompounds and the other of the first and second semiconductor layers isformed of materials selected from the group of II-VI elements.
 102. Themethod of forming a photovoltaic light-to-electrical energy transducerincluding a thin-film A-B-type heterojunction as set forth in claims 93,94 or 95 wherein the ternary semiconductor material is CuInSe₂.
 103. Themethod of forming a photovoltaic light-to-electrical energy transducerincluding a thin-film A-B-type heterojunction as set forth in claims 93,94 or 95 wherein the other of the first and second semiconductor layershas a band gap energy greater than 1.5 ev.
 104. The method of forming aphotovoltaic light-to-electrical energy transducer including a thin-filmA-B-type heterojunction as set forth in claims 93, 94 or 95 wherein theother of the first and second semiconductor layers is selected from thegroup consisting of:CdS; Cd_(1--x) Zn_(x) S; and, CdS_(1--x) Se_(x).105. The method of forming a photovoltaic light-to-electrical energytransducer including a thin-film A-B-type heterojunction as set forth inclaims 93, 94 or 95 wherein the ternary semiconductor material isCuInSe₂ and the other of the first and second semiconductor layers isselected from the group consisting of:CdS; Cd_(1--x) Zn_(x) S; and,CdS_(1--x) Se_(x).
 106. The method of forming a photovoltaiclight-to-electrical energy transducer including a thin-film A-B-typeheterojunction as set forth in claims 93, 94 or 95 wherein the ternarysemiconductor material is CuInSe₂ and the other of the first and secondsemiconductor layers is CdS.
 107. The method of forming a photovoltaiclight-to-electrical energy transducer including a thin-film A-B-typeheterojunction as set forth in claims 93, 94 or 95 wherein the ternarysemiconductor material is a I-III-VI₂ chalcopyrite compound and theratio of the I-III-elements is adjusted during one portion only ofsimultaneous elemental evaporation of the ternary compound so as to formone of the low resistivity and transient high resistivity regions of thetransient homojunction, and adjusted to a different I-III ratio duringthe remaining portion of simultaneous elemental evaporation of theternary compound so as to form the other of the low resistivity and thetransient high resistivity regions of the transient homojunction. 108.The method of forming a photovoltaic light-to-electrical energytransducer including a thin-film A-B-type heterojunction as set forth inclaims 93, 94 or 95 wherein the ternary semiconductor layer is CuInSe₂and the copper/indium ratio is initially adjusted to form a slightlycopper-enriched region during simultaneous elemental evaporation of theCuInSe₂ to form the low resistivity region of the transducer of p-typematerial, and is readjusted to form a slightly copper-deficient regionduring simultaneous elemental evaporation of the CuInSe₂ to form thehigh resistivity transient semiconductor region of the transducer oftransient n-type material.
 109. The product produced by the method setforth in claims 93, 94 or
 95. 110. The method of forming a p-n-typeheterojunction photovoltaic device comprising the steps of:(a)depositing a first region of relatively low resistivity p-type materialon a metallized substrate; (b) depositing a second region of relativelyhigh resistivity transient n-type material formed of the same elementalconstituents as the relatively low resistivity p-type material depositedin step (a) with such transient n-type material being deposited on thefirst region of p-type material and defining therewith a transientp-n-type homojunction; (c) depositing a film of low resistivity n-typesemiconductor material on the transient p-n-type homojunction formed insteps (a) and (b); and, (d) heating the p-n-type heterojunctionphotovoltaic device formed in steps (a), (b) and (c); whereuponinterdiffusion of the constituent elements of the materials employed insteps (a), (b) and (c) between the p-type material and the transientn-type material, and between the transient n-type material and then-type semiconductor material, causes the transient n-type material toevolve into relatively high resistivity p-type material so as to form athin-film heterojunction essentially devoid of growth nodules andproviding a photovoltaic response characteristic of energy transducershaving relatively high conversion efficiencies.
 111. The method of claim110 wherein the device formed is heated during step (d) in the presenceof air.
 112. The method of claim 110 wherein the device formed is heatedduring step (d) in the presence of H₂ /Ar and air.
 113. The method ofclaims 110, 111 or 112 wherein the device formed is heated during step(d) at a temperature on the order of 200°C.
 114. The method of claims110, 111 or 112 wherein the device formed is heated during step (d) at atemperature on the order of 200° C. for a period on the order of 20minutes.
 115. The method of claim 110 wherein the first and secondregions of p-type and transient n-type material, respectively, comprisea ternary semiconductor material formed by simultaneous elementalevaporation.
 116. The method of claim 115 wherein the ternarysemiconductor material is a chalcopyrite compound.
 117. The method ofclaim 115 wherein the ternary semiconductor material is a materialselected from the class of I-III-VI₂ chalcopyrite compounds.
 118. Themethod of claim 115 wherein the ternary semiconductor material isCuInSe₂.
 119. The method of claim 118 wherein the copper/indium ratio inthe ternary semiconductor material is initially adjusted to form aslightly copper-enriched region during simultaneous elementalevaporation thereof to form the first region of relatively lowresistivity p-type material in step (a) and is readjusted during suchsimultaneous elemental evaporation to form a slightly copper-deficientternary compound during evaporation of the second region of relativelyhigh resistivity transient n-type material formed in step (b).
 120. Themethod of claim 118 wherein the copper/indium ratio is altered uponformation of at least about 50% of the total desired thickness of thetransient p-n-type homojunction but prior to formation of about 66.7% ofthe total desired thickness of the transient p-n-type homojunction. 121.The method of claim 118 wherein the film of low resistivity n-typesemiconductor material is formed of n-type material having a band gapenergy greater than 1.5 ev.
 122. The method of claim 121 wherein then-type low resistivity semiconductor material is a II-VI material. 123.The method of claim 127 wherein the II-VI material is selected from thegroup consisting of:CdS; Cd_(1--x) Zn_(x) S; and, CdS_(1--x) Se_(x).124. The method of claim 122 wherein the II-VI material is CdS.
 125. Themethod of claim 110 wherein the film of low-resistivity n-typesemiconductor material comprises a first region of relatively pure CdSand a second superimposed region of indium-doped CdS.
 126. The method ofclaim 110 wherein the first and second regions of semiconductor materialformed in steps (a) and (b) are formed of a temperature in the range of350° C. to 500° C.
 127. The method of claim 110 wherein the first regionof semiconductor material formed in step (a) and a portion of the secondregion of semiconductor material formed in step (b) are formed at atemperature in the range of 350° C. to a temperature less than 450° C.,and the remainder of the second region of semiconductor material formedin step (b) is formed at a temperature in the range of about 450° to500° C.
 128. The method of claim 110 wherein the first region ofsemiconductor material formed in step (a) and a portion of the secondregion of semiconductor material formed in step (b) are formed at atemperature on the order of 350° C., and the remainder of the secondregion of semiconductor material formed in step (b) is formed at atemperature on the order of 450° C.±approximately 25° C.
 129. The methodof claim 110 wherein the film of low resistivity n-type semiconductormaterial formed in step (c) is formed at a temperature in the range of150° C. to 200° C.
 130. The method of claims 126, 127 or 128 wherein thefilm of low resistivity n-type semiconductor material formed in step (c)is formed at a temperature in the range of 150° C. to 200° C.
 131. Themethod of claims 110, 116, 117, 118, 124, 132 or 133 wherein the firstand second regions of semiconductor material formed in steps (a) and (b)are formed in an atmosphere maintained at 3-8×10⁻⁶ torr.
 132. The methodof claims 110, 116, 117 or 118 wherein a grid-like contact is formed onthe film of low resistivity n-type semiconductor material deposited instep (c) and an antireflective coating is formed on the grid-likecontact and the exposed surface of the film of low resistivity n-typesemiconductor material.
 133. The method of claims 110, 115, 116, 117 or118 wherein a grid-like contact is formed on the film of low resistivityn-type semiconductor material deposited in step (c) and anantireflective coating formed of SiO_(x) is formed on the grid-likecontact and the exposed surface of the film of low resistivity n-typesemiconductor material.
 134. The method of claims 110, 111, 117 or 118wherein a grid-like contact is formed on the film of low resistivityn-type semiconductor material deposited in step (c) and anantireflective coating formed of SiO_(x) wherein "x" is on the order of"1.8" is formed on the grid-like contact and the exposed surface of thefilm of low resistivity n-type semiconductor material.
 135. The productproduced by the method set forth in claims 110, 115, 116, 117, 118, 119,127, 128 or
 129. 136. A thin-film A-B type heterojunction photovoltaicdevice wherein "A" and "B" are selected from the group of semiconductormaterials consisting of:

    ______________________________________                                        A                  AND     B                                                  ______________________________________                                        (1)   a p-type ternary material                                                                      and     an n-type material; or,                        (2)   an n-type ternary material                                                                     and     a p-type material; or,                         (3)   an n-type material                                                                             and     a p-type ternary material; or,                 (4)   a p-type material                                                                              and     an n-type ternary                                                             material;                                      ______________________________________                                    

comprising a first semiconductor layer .[.having been formed with afirst region of A-type material and a second superimposed region oftransient B-type material with said first and second regions initiallydefining a transient A-B-type homojunction, and a second semiconductorlayer deposited on said first layer and formed of a second B-typematerial whereupon interdiffusion of the constituent elements defining:(i) said A-type material; (ii) said transient B-type material; and(iii), said second B-type material; causes the transient B-type materialto evolve into A-type material so as to form a thin-film.]. .Iadd.formedof A-type material and a second superimposed semiconductor layer formedof B-type material deposited on said first layer; one of said first andsecond semiconductor layers having been formed with a first region ofternary material comprising a selected one of a p-type ternary materialor an n-type ternary material, and a second transient region of ternarymaterial of the opposite conductivity type as that selected in saidfirst region, said second transient region of ternary material beingadjacent the other of said first and second semiconductor layers, withsaid first and second regions of said one of said first and secondsemiconductor layers initially defining a transient homojunctioncomprising one of a p-n-type or an n-p-type homojunction; whereuponinterdiffusion of the constituent elements defining: (i) said firstregion of ternary material in said one of said first and secondsemiconductor layers respectively formed of A-type and B-type materials;(ii) said second transient region of ternary material in said one ofsaid first and second semiconductor layers; and (iii), the other of saidfirst and second semiconductor layers; causes the second transientregion of ternary material to evolve into a ternary material having thesame p-type of n-type characteristic as said first region of ternarymaterial so as to form a thin-film A-B-type .Iaddend.heterojunctionpermitting a photovoltaic response characteristic of an energytransducer capable of exhibiting a conversion efficiency approximatingon the order of 10%.
 137. The thin-film A-B-type heterojunctionphotovoltaic device set forth in claim 136 wherein said A-B-typeheterojunction is a p-n-type heterojunction with said firstsemiconductor layer defining said transient .[.A-B-.]. homojunctionhaving been formed as a transient p-n-type homojunction having a.Iadd.first .Iaddend.region of low resistivity p-type material and a.Iadd.second .Iaddend.region of high resistivity transient n-typematerial, and said second semiconductor layer is formed of n-typesemiconductor material deposited in face-to-face contact with said.Iadd.second transient .Iaddend.region of high resistivity transientn-type material.
 138. The thin-film A-B-type heterojunction photovoltaicdevice as set forth in claim 136 wherein said A-B-type heterojunction isan n-p-type heterojunction with said first semiconductor layer definingsaid transient .[.A-B.]. homojunction having been formed as a transientn-p-type homojunction having a .[.region of high resistivity transientn-type material and a region of low resistivity.]. .Iadd.first region oflow resistivity n-type material and a second region of high resistivitytransient .Iaddend.p-type material, and said second semiconductor layeris formed of .[.n-type.]. .Iadd.p-type .Iaddend.semiconductor materialdeposited in face-to-face contact with said .Iadd.second transient.Iaddend.region of high resistivity transient .[.n-type.]. .Iadd.p-type.Iaddend.material.
 139. The thin-film A-B-type heterojunctionphotovoltaic .[.devive.]. .Iadd.device .Iaddend.as set forth in claim136 wherein .[.said first semiconductor layer is formed of a.]..Iadd.one of said first and second semiconductor layers is formed offirst and second regions of .Iaddend.ternary material selected from thegroup of I-III-VI₂ chalcopyrite compounds.
 140. The A-B-typeheterojunction photovoltaic device as set forth in claim 136 wherein.[.said first semiconductor layer is formed of a.]. .Iadd.one of saidfirst and second semiconductor layers is formed of first and secondregions of .Iaddend.ternary material selected from the group ofI-III-VI₂ chalcopyrite compounds and .[.said second semiconductorlayer.]. .Iadd.the other of said first and second semiconductor layers.Iaddend.is formed of materials selected from the group of II-VIelements.
 141. The thin-film A-B-type heterojunction photovoltaic deviceas set forth in claims 139 or 140 wherein said ternary material has aband gap energy in the range of 1-1.5 ev.
 142. The thin-film A-B-typeheterojunction photovoltaic device as set forth in claims 139 or 140wherein said ternary material has a band gap energy in the range of1-1.5 ev and .[.said second semiconductor layer is.]. .Iadd.the other ofsaid first and second semiconductor layers is .Iaddend.formed ofmaterials having a band gap energy greater than 1.5 ev.
 143. Thethin-film A-B-type heterojunction photovoltaic device as set forth inclaims 136, 137 or 138 wherein said g .[.first semiconductor layer isformed of CuInSe₂..]. .Iadd.ternary material is CuInSe₂. .Iaddend. 144.The thin-film A-B-type heterojunction photovoltaic device as set forthin claims .[.141, 142 or 143.]. .Iadd.136, 137 or 138 .Iaddend.wherein.[.said second semiconductor layer.]. .Iadd.the other of said first andsecond semiconductor layers .Iaddend.has a band gap energy greater than1.5 ev.
 145. The thin-film A-B-type heterojunction photovoltaic deviceas set forth in claims 136, 137 or 138 wherein .[.said secondsemiconductor layer.]. .Iadd.the other of said first and secondsemiconductor layers .Iaddend.is formed of materials selected from thegroup consisting of:CdS; Cd_(1--x) Zn_(x) S; and, CdS_(1--x) Se_(x).146. The thin-film A-B-type heterojunction photovoltaic device as setforth in claims 136, 137 or 138 wherein .[.said first semiconductor isformed of a ternary semiconductor material comprising CuInSe₂ and saidsecond semiconductor layer.]. .Iadd.one of said first and secondsemiconductor layers is formed of a ternary semiconductor materialcomprising CuInSe₂ and the other of said first and second semiconductorlayers .Iaddend.is formed of materials selected from the groupconsisting of:CdS; Cd_(1--x) Zn_(x) S; and, CdS_(1--x) Se_(x).
 147. Thethin-film A-B-type heterojunction photovoltaic device as set forth inclaims 136, 137 or 138 wherein .[.said first semiconductor layer isformed of a ternary semiconductor material comprising CuInSe₂ and saidsecond semiconductor layer is CdS..]. .Iadd.one of said first and secondsemiconductor layers is formed of a ternary semiconductor materialcomprising CuInSe₂ and the other of said first and second semiconductorlayers is CdS. .Iaddend.